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Binary-Indexed-Tree

Segment Tree and Fenwick Tree

2026-10-21·3 mins

Algorithm Segment-Tree Binary-Indexed-Tree Design

Point updates with range queries: a Fenwick tree (binary indexed tree) jumps by i & -i, and a segment tree recursively halves the range; both take O(log n) per operation

LeetCode 4051: Count Subarrays with Distant Sums

2026-10-19·5 mins

LeetCode Hard Merge-Sort Prefix-Sum Binary-Indexed-Tree Biweekly-Contest

LeetCode 4051 note: prefix sums, complement counting, range counting, and the k = 0 edge case

LeetCode 315: Count of Smaller Numbers After Self

2026-10-18·4 mins

LeetCode Hard Merge-Sort Divide-and-Conquer Binary-Indexed-Tree Counting

LeetCode 315 note: merge sort with indices, plus a BIT solution

LeetCode 2193: Minimum Number of Moves to Make Palindrome

2026-09-02·3 mins

LeetCode Hard Two-Pointers Greedy String Binary-Indexed-Tree

LeetCode 2193 note: greedily fix the outer pair: take the last character and swap its leftmost partner to the front, costing that index; the unpaired character ends in the middle, costing its distance there

LeetCode 327: Count of Range Sum

2026-08-01·3 mins

LeetCode Hard Merge-Sort Divide-and-Conquer Prefix-Sum Binary-Indexed-Tree

LeetCode 327 note: prefix sums plus merge sort counting with two pointers bounding a range, and a log of retyping it over a dozen times

LeetCode 307: Range Sum Query - Mutable

2026-06-19·3 mins

LeetCode Medium Segment-Tree Binary-Indexed-Tree Design Array

LeetCode 307 note: point updates with range sums make prefix sums O(n) per update; a segment tree or Fenwick tree makes both operations O(log n)