Tags

TODO

• [CodeChef] Starters 84 — SUM OR: a correct digit DP that still TLEs
• [CodeChef] Starters 88 — Chef and Good Array: pairs are intervals
• [CodeChef] Starters 88 — Minimize the Bits: the one I couldn't solve

ad_hoc

• [CodeChef] Starters 117 — Equality Etiquette: the sign choice is the whole problem
• [CodeChef] Starters 253 — Flower Reversal: only the two boundaries move
• [CodeChef] Starters 96 — Zero Array: when the last equation is the whole problem

amortized_analysis

• [AtCoder] ABC470 C — Inc, Dec, Xor: the "pay with tokens you already minted" trick

arrays

• Two Pointers
• [CodeChef] Starters 108 — Clan Expansion: the largest gap between sources

atcoder

• [AtCoder] ABC 476 D — Automat: equal money, unequal wallets
• [AtCoder] ABC466 C — Count Close Pairs
• [AtCoder] ABC469 E — Pro Exam Eligibility
• [AtCoder] ABC470 C — Inc, Dec, Xor: the "pay with tokens you already minted" trick
• [AtCoder] ABC471 D — Chargers: subtract the common term
• [AtCoder] ABC472 E — Odd Cycle: depth parity, and two bugs in reconstructing the cycle
• [AtCoder] ARC226 A — Meeting Division: when the constraint is the solution

backtracking

• Backtracking — The Art of Systematic Undoing

bfs

• [CodeChef] Starters 106 — Reach Anywhere: finding shortest odd and even parity distances
• [CodeChef] Starters 116 — Expected Diameter: two extra nodes and the vertices that matter

bfs_tree

• [AtCoder] ABC472 E — Odd Cycle: depth parity, and two bugs in reconstructing the cycle

binary_search

• [AtCoder] ABC469 E — Pro Exam Eligibility
• [CodeChef] Starters 115 — Make All Zero: Only Prefix Minima Can Be Eliminated
• [CodeChef] Starters 253 — Dis-Card: my dominance count was off by one, and off by a lot
• [CodeChef] Starters 89 — Two Averages: fix the total, then split it
• [CodeChef] Starters 97 — Triplets Min: the binary search that collapses into a prefix sum
• [LeetCode] Weekly Contest 514 — Maximum Area of Two Non-Overlapping Square Submatrices: only the extremes can witness a valid pair
• [LeetCode] Weekly Contest 515 — Maximum Gap Between Stations: greedy extremes, and the max-of-max trap
• [Leetcode] Biweekly 188 — Minimum Possible Maximum Waiting Time
• [Repovive] Starter Round 4 D — Distant Transfers: Deriving Invariants Instead of Constructing Moves

bipartite

• [AtCoder] ABC472 E — Odd Cycle: depth parity, and two bugs in reconstructing the cycle
• [CodeChef] Starters 96 — Zero Array: when the last equation is the whole problem

bit_manipulation

• [CodeChef] Starters 84 — SUM OR: a correct digit DP that still TLEs
• [CodeChef] Starters 88 — Minimize the Bits: the one I couldn't solve
• [Codeforces] Round 2239 D1 — XOR Sorting (Easy)

bitmask

• [CodeChef] Starters 93 — Greedy: the problem is called Greedy and the answer is a DP
• [LeetCode] Weekly Contest 515 — Elevator Requests III: Held–Karp, a poisoned sentinel, and a duplicate-floor scare

bitmasks

• [yukicoder] Contest 509 — Product: the answer is always 2^k times 2^k-1

brute_force

• [CodeChef] Starters 253 — Grid Jump: I assumed the answer sits at a corner
• [Codeforces] Round 1103 (Div. 3) E — Friendly Gifts: Disjoint Value Intervals

codechef

• [CodeChef] FALLPR — Fall Prevention: I deleted the wrong element
• [CodeChef] SHSC — Shift Score: I reached for rerooting when counting edges was enough
• [CodeChef] Starters 105 — Wishcraft
• [CodeChef] Starters 106 — Reach Anywhere: finding shortest odd and even parity distances
• [CodeChef] Starters 108 — Clan Expansion: the largest gap between sources
• [CodeChef] Starters 115 — Make All Zero: Only Prefix Minima Can Be Eliminated
• [CodeChef] Starters 116 — Expected Diameter: two extra nodes and the vertices that matter
• [CodeChef] Starters 117 — Equality Etiquette: the sign choice is the whole problem
• [CodeChef] Starters 247 — Fair Flipping (Easy): What This Constructive Proof Taught Me
• [CodeChef] Starters 247 — Red Blue Swaps: From Swaps to Buckets (A DP Pattern)
• [CodeChef] Starters 250 — Subsequence 1: Chains, not cuts
• [CodeChef] Starters 250 — Subsequence 2: Counting windows, one threshold at a time
• [CodeChef] Starters 252 — Tree Counting: a parity invariant, Scoins' formula, and a DP state I got wrong
• [CodeChef] Starters 253 — Dis-Card: my dominance count was off by one, and off by a lot
• [CodeChef] Starters 253 — Flower Reversal: only the two boundaries move
• [CodeChef] Starters 253 — Grid Jump: I assumed the answer sits at a corner
• [CodeChef] Starters 81 — Beautiful Strings: the DP state I didn't need
• [CodeChef] Starters 81 — Good XOR: the case I ruled out with a parity argument
• [CodeChef] Starters 84 — SUM OR: a correct digit DP that still TLEs
• [CodeChef] Starters 86 — Minimum Operation: the case my gut missed
• [CodeChef] Starters 87 — Count the Permutations (always)
• [CodeChef] Starters 88 — Chef and Good Array: pairs are intervals
• [CodeChef] Starters 88 — Minimize the Bits: the one I couldn't solve
• [CodeChef] Starters 89 — Two Averages: fix the total, then split it
• [CodeChef] Starters 90 — Minimum Ugliness: the diameter of a subset
• [CodeChef] Starters 93 — Greedy: the problem is called Greedy and the answer is a DP
• [CodeChef] Starters 93 — Thank U, Next: dijkstra on energy graph
• [CodeChef] Starters 95 — Break This Array: the cut probability I kept forgetting
• [CodeChef] Starters 96 — Zero Array: when the last equation is the whole problem
• [CodeChef] Starters 97 — Triplets Min: the binary search that collapses into a prefix sum

codeforces

• [Codeforces] Edu Round 192 B — A Small Algebra Trick That Turns an O(n²) Idea into O(n)
• [Codeforces] Edu Round 192 D — From Merging Digits to Longest Common Subsequence
• [Codeforces] Round 1103 (Div. 3) E — Friendly Gifts: Disjoint Value Intervals
• [Codeforces] Round 1103 (Div. 3) F1 — Elections in Saransk (Easy Version): Prime-wise Counting
• [Codeforces] Round 1103 (Div. 3) F2 — Elections in Saransk (Hard Version): Sum-minus-Max DP
• [Codeforces] Round 179 (Div. 1) B — Greg and Graph: Learning to Think in Reverse
• [Codeforces] Round 2239 D1 — XOR Sorting (Easy)
• [Codeforces] Round 267 (Div. 2) C — George and Job: DP on Fixed-Length Segments

combinatorics

• [CodeChef] Starters 81 — Beautiful Strings: the DP state I didn't need
• [CodeChef] Starters 84 — SUM OR: a correct digit DP that still TLEs
• [CodeChef] Starters 87 — Count the Permutations (always)
• [CodeChef] Starters 97 — Triplets Min: the binary search that collapses into a prefix sum

complexity_analysis

• [Leetcode] Biweekly 188 — Fence Width Optimization: From O(n³) to O(n²)

constructive

• [CodeChef] Starters 117 — Equality Etiquette: the sign choice is the whole problem
• [CodeChef] Starters 96 — Zero Array: when the last equation is the whole problem

constructive_algorithms

• [CodeChef] Starters 247 — Fair Flipping (Easy): What This Constructive Proof Taught Me

contribution_technique

• [CodeChef] SHSC — Shift Score: I reached for rerooting when counting edges was enough

convolution

• Number Theoretic Transform

counting

• [CodeChef] Starters 116 — Expected Diameter: two extra nodes and the vertices that matter
• [CodeChef] Starters 250 — Subsequence 2: Counting windows, one threshold at a time
• [CodeChef] Starters 252 — Tree Counting: a parity invariant, Scoins' formula, and a DP state I got wrong
• [CodeChef] Starters 81 — Beautiful Strings: the DP state I didn't need
• [CodeChef] Starters 87 — Count the Permutations (always)
• [Leetcode] Biweekly 188 — Fence Width Optimization: From O(n³) to O(n²)

cp_cases

• [CodeChef] Starters 253 — Flower Reversal: only the two boundaries move

cycles

• [AtCoder] ABC472 E — Odd Cycle: depth parity, and two bugs in reconstructing the cycle

dfs

• Backtracking — The Art of Systematic Undoing

digit_dp

• [CodeChef] Starters 84 — SUM OR: a correct digit DP that still TLEs

dijkstra

• [CodeChef] Starters 93 — Thank U, Next: dijkstra on energy graph

divisibility

• Number Theory

dynamic_programming

• Dynamic Programming
• [CodeChef] Starters 247 — Red Blue Swaps: From Swaps to Buckets (A DP Pattern)
• [CodeChef] Starters 250 — Subsequence 1: Chains, not cuts
• [CodeChef] Starters 250 — Subsequence 2: Counting windows, one threshold at a time
• [CodeChef] Starters 252 — Tree Counting: a parity invariant, Scoins' formula, and a DP state I got wrong
• [CodeChef] Starters 81 — Beautiful Strings: the DP state I didn't need
• [CodeChef] Starters 84 — SUM OR: a correct digit DP that still TLEs
• [CodeChef] Starters 88 — Chef and Good Array: pairs are intervals
• [CodeChef] Starters 93 — Greedy: the problem is called Greedy and the answer is a DP
• [CodeChef] Starters 95 — Break This Array: the cut probability I kept forgetting
• [Codechef] Starters 248 — Merging Parity
• [Codeforces] Edu Round 192 D — From Merging Digits to Longest Common Subsequence
• [Codeforces] Round 1103 (Div. 3) F2 — Elections in Saransk (Hard Version): Sum-minus-Max DP
• [Codeforces] Round 267 (Div. 2) C — George and Job: DP on Fixed-Length Segments
• [LeetCode] Weekly Contest 514 — Maximum Area of Two Non-Overlapping Square Submatrices: only the extremes can witness a valid pair
• [LeetCode] Weekly Contest 515 — Elevator Requests III: Held–Karp, a poisoned sentinel, and a duplicate-floor scare
• [Leetcode] Biweekly 186 — Count Distinct Ways to Form Target from Two Strings
• [Leetcode] Biweekly 188 — Minimum Possible Maximum Waiting Time

expected_value

• [CodeChef] Starters 95 — Break This Array: the cut probability I kept forgetting

fft

• Number Theoretic Transform

floyd_warshall

• [Codeforces] Round 179 (Div. 1) B — Greg and Graph: Learning to Think in Reverse

gcd

• [CodeChef] Starters 86 — Minimum Operation: the case my gut missed

graph

• [CodeChef] Starters 93 — Thank U, Next: dijkstra on energy graph

graph_theory

• [AtCoder] ARC226 A — Meeting Division: when the constraint is the solution
• [CodeChef] Starters 116 — Expected Diameter: two extra nodes and the vertices that matter
• [CodeChef] Starters 90 — Minimum Ugliness: the diameter of a subset

graphs

• [AtCoder] ABC472 E — Odd Cycle: depth parity, and two bugs in reconstructing the cycle
• [CodeChef] Starters 106 — Reach Anywhere: finding shortest odd and even parity distances
• [Codeforces] Round 179 (Div. 1) B — Greg and Graph: Learning to Think in Reverse
• [Codeforces] Round 2239 D1 — XOR Sorting (Easy)

greedy

• [AtCoder] ABC 476 D — Automat: equal money, unequal wallets
• [CodeChef] FALLPR — Fall Prevention: I deleted the wrong element
• [CodeChef] Starters 105 — Wishcraft
• [CodeChef] Starters 108 — Clan Expansion: the largest gap between sources
• [CodeChef] Starters 115 — Make All Zero: Only Prefix Minima Can Be Eliminated
• [CodeChef] Starters 250 — Subsequence 1: Chains, not cuts
• [CodeChef] Starters 253 — Dis-Card: my dominance count was off by one, and off by a lot
• [CodeChef] Starters 253 — Flower Reversal: only the two boundaries move
• [CodeChef] Starters 253 — Grid Jump: I assumed the answer sits at a corner
• [CodeChef] Starters 81 — Good XOR: the case I ruled out with a parity argument
• [CodeChef] Starters 88 — Chef and Good Array: pairs are intervals
• [CodeChef] Starters 88 — Minimize the Bits: the one I couldn't solve
• [CodeChef] Starters 89 — Two Averages: fix the total, then split it
• [LeetCode] Weekly Contest 515 — Elevator Requests III: Held–Karp, a poisoned sentinel, and a duplicate-floor scare
• [LeetCode] Weekly Contest 515 — Maximum Gap Between Stations: greedy extremes, and the max-of-max trap
• [Repovive] Starter Round 4 D — Distant Transfers: Deriving Invariants Instead of Constructing Moves
• [yukicoder] Contest 509 — Product: the answer is always 2^k times 2^k-1

hash_maps

• [Leetcode] Biweekly 188 — Fence Width Optimization: From O(n³) to O(n²)

hashing

• [Codeforces] Round 1103 (Div. 3) E — Friendly Gifts: Disjoint Value Intervals

heap

• [AtCoder] ABC471 D — Chargers: subtract the common term

interactive

• [AtCoder] ABC466 C — Count Close Pairs

interval_graphs

• [AtCoder] ARC226 A — Meeting Division: when the constraint is the solution

intervals

• [CodeChef] Starters 88 — Chef and Good Array: pairs are intervals

invariants

• [AtCoder] ABC471 D — Chargers: subtract the common term
• [CodeChef] Starters 252 — Tree Counting: a parity invariant, Scoins' formula, and a DP state I got wrong
• [CodeChef] Starters 81 — Good XOR: the case I ruled out with a parity argument
• [Codechef] Starters 248 — Deleting Elements (Easy)
• [LeetCode] Weekly Contest 514 — Maximum Area of Two Non-Overlapping Square Submatrices: only the extremes can witness a valid pair

lca

• [CodeChef] Starters 90 — Minimum Ugliness: the diameter of a subset

lcs

• [Codeforces] Edu Round 192 D — From Merging Digits to Longest Common Subsequence

leetcode

• [LeetCode] Weekly Contest 514 — Maximum Area of Two Non-Overlapping Square Submatrices: only the extremes can witness a valid pair
• [LeetCode] Weekly Contest 515 — Elevator Requests III: Held–Karp, a poisoned sentinel, and a duplicate-floor scare
• [LeetCode] Weekly Contest 515 — Maximum Gap Between Stations: greedy extremes, and the max-of-max trap
• [LeetCode] Weekly Contest 516 — Valid K-Unique Subarrays I: Mo's algorithm, and where the √N actually comes from
• [Leetcode] Biweekly 186 — Count Distinct Ways to Form Target from Two Strings
• [Leetcode] Biweekly 188 — Fence Width Optimization: From O(n³) to O(n²)
• [Leetcode] Biweekly 188 — Minimum Possible Maximum Waiting Time
• [Leetcode] Weekly 509 — Palindromic Subarray Sum with Rolling Hashes
• [Leetcode] Weekly 509 — Subsequence After One Replacement: Did You Consume the Matched Character?

linear_algebra

• [CodeChef] Starters 96 — Zero Array: when the last equation is the whole problem

math

• [CodeChef] Starters 117 — Equality Etiquette: the sign choice is the whole problem
• [CodeChef] Starters 253 — Grid Jump: I assumed the answer sits at a corner
• [CodeChef] Starters 89 — Two Averages: fix the total, then split it
• [Repovive] Starter Round 4 C — Corner Meeting: Minimizing the Max of an Increasing and a Decreasing Function

missed_case

• [CodeChef] Starters 81 — Good XOR: the case I ruled out with a parity argument

mobius

• Möbius Function

modular_arithmetic

• [CodeChef] Starters 95 — Break This Array: the cut probability I kept forgetting

mos_algorithm

• [LeetCode] Weekly Contest 516 — Valid K-Unique Subarrays I: Mo's algorithm, and where the √N actually comes from

new_pattern

• [CodeChef] Starters 81 — Beautiful Strings: the DP state I didn't need

ntt

• Number Theoretic Transform

number_theory

• Dynamic Programming
• Möbius Function
• Number Theory
• [CodeChef] Starters 86 — Minimum Operation: the case my gut missed
• [Codeforces] Round 1103 (Div. 3) F1 — Elections in Saransk (Easy Version): Prime-wise Counting
• [Codeforces] Round 1103 (Div. 3) F2 — Elections in Saransk (Hard Version): Sum-minus-Max DP

numpy

• [CodeChef] Starters 90 — Minimum Ugliness: the diameter of a subset

offline_queries

• [LeetCode] Weekly Contest 516 — Valid K-Unique Subarrays I: Mo's algorithm, and where the √N actually comes from

palindromes

• Number Theory

parity

• [CodeChef] Starters 117 — Equality Etiquette: the sign choice is the whole problem
• [CodeChef] Starters 253 — Grid Jump: I assumed the answer sits at a corner
• [CodeChef] Starters 81 — Good XOR: the case I ruled out with a parity argument
• [CodeChef] Starters 96 — Zero Array: when the last equation is the whole problem

pending

• [CodeChef] Starters 253 — Dis-Card: my dominance count was off by one, and off by a lot

permutations

• [CodeChef] Starters 253 — Dis-Card: my dominance count was off by one, and off by a lot
• [CodeChef] Starters 87 — Count the Permutations (always)

polynomials

• Number Theoretic Transform

prefix_sums

• [AtCoder] ABC 476 D — Automat: equal money, unequal wallets
• [AtCoder] ABC469 E — Pro Exam Eligibility
• [AtCoder] ARC226 A — Meeting Division: when the constraint is the solution
• [CodeChef] FALLPR — Fall Prevention: I deleted the wrong element
• [CodeChef] Starters 97 — Triplets Min: the binary search that collapses into a prefix sum
• [Codeforces] Edu Round 192 B — A Small Algebra Trick That Turns an O(n²) Idea into O(n)
• [LeetCode] Weekly Contest 514 — Maximum Area of Two Non-Overlapping Square Submatrices: only the extremes can witness a valid pair

probability

• [CodeChef] Starters 95 — Break This Array: the cut probability I kept forgetting

problem_solving

• [Codechef] Starters 248 — Deleting Elements (Easy)
• [Codechef] Starters 248 — Merging Parity

python

• [AtCoder] ABC 476 D — Automat: equal money, unequal wallets
• [CodeChef] FALLPR — Fall Prevention: I deleted the wrong element
• [CodeChef] Starters 81 — Beautiful Strings: the DP state I didn't need
• [CodeChef] Starters 81 — Good XOR: the case I ruled out with a parity argument
• [CodeChef] Starters 84 — SUM OR: a correct digit DP that still TLEs
• [CodeChef] Starters 86 — Minimum Operation: the case my gut missed
• [CodeChef] Starters 87 — Count the Permutations (always)
• [CodeChef] Starters 88 — Chef and Good Array: pairs are intervals
• [CodeChef] Starters 88 — Minimize the Bits: the one I couldn't solve
• [CodeChef] Starters 89 — Two Averages: fix the total, then split it
• [CodeChef] Starters 90 — Minimum Ugliness: the diameter of a subset
• [CodeChef] Starters 93 — Greedy: the problem is called Greedy and the answer is a DP
• [CodeChef] Starters 93 — Thank U, Next: dijkstra on energy graph

recursion

• Backtracking — The Art of Systematic Undoing

repovive

• [Repovive] Starter Round 4 C — Corner Meeting: Minimizing the Max of an Increasing and a Decreasing Function
• [Repovive] Starter Round 4 D — Distant Transfers: Deriving Invariants Instead of Constructing Moves

rolling_hashing

• [Leetcode] Weekly 509 — Palindromic Subarray Sum with Rolling Hashes

rust

• [AtCoder] ABC470 C — Inc, Dec, Xor: the "pay with tokens you already minted" trick

scheduling

• [Leetcode] Biweekly 188 — Minimum Possible Maximum Waiting Time

shortest_paths

• [CodeChef] Starters 106 — Reach Anywhere: finding shortest odd and even parity distances
• [CodeChef] Starters 93 — Thank U, Next: dijkstra on energy graph
• [Codeforces] Round 179 (Div. 1) B — Greg and Graph: Learning to Think in Reverse

sieve

• [CodeChef] Starters 86 — Minimum Operation: the case my gut missed

silly_mistake

• [CodeChef] FALLPR — Fall Prevention: I deleted the wrong element
• [CodeChef] Starters 253 — Grid Jump: I assumed the answer sits at a corner

sorting

• [CodeChef] Starters 105 — Wishcraft
• [CodeChef] Starters 97 — Triplets Min: the binary search that collapses into a prefix sum

sparse_table

• [CodeChef] Starters 253 — Dis-Card: my dominance count was off by one, and off by a lot

sqrt_decomposition

• [LeetCode] Weekly Contest 516 — Valid K-Unique Subarrays I: Mo's algorithm, and where the √N actually comes from

strings

• [CodeChef] Starters 253 — Flower Reversal: only the two boundaries move
• [CodeChef] Starters 93 — Greedy: the problem is called Greedy and the answer is a DP

subset_sum

• [CodeChef] Starters 93 — Greedy: the problem is called Greedy and the answer is a DP

todo

• [CodeChef] SHSC — Shift Score: I reached for rerooting when counting edges was enough
• [CodeChef] Starters 95 — Break This Array: the cut probability I kept forgetting

trees

• [CodeChef] SHSC — Shift Score: I reached for rerooting when counting edges was enough
• [CodeChef] Starters 116 — Expected Diameter: two extra nodes and the vertices that matter
• [CodeChef] Starters 252 — Tree Counting: a parity invariant, Scoins' formula, and a DP state I got wrong
• [CodeChef] Starters 90 — Minimum Ugliness: the diameter of a subset

two_pointers

• Two Pointers
• [AtCoder] ABC 476 D — Automat: equal money, unequal wallets
• [AtCoder] ABC466 C — Count Close Pairs
• [AtCoder] ABC469 E — Pro Exam Eligibility
• [LeetCode] Weekly Contest 515 — Maximum Gap Between Stations: greedy extremes, and the max-of-max trap
• [Leetcode] Weekly 509 — Subsequence After One Replacement: Did You Consume the Matched Character?

unnessary_introduction_of_complexity

• [CodeChef] SHSC — Shift Score: I reached for rerooting when counting edges was enough

xor

• [AtCoder] ABC470 C — Inc, Dec, Xor: the "pay with tokens you already minted" trick
• [Codeforces] Round 2239 D1 — XOR Sorting (Easy)

yukicoder

• [yukicoder] Contest 509 — Product: the answer is always 2^k times 2^k-1