Two-Pointers

LeetCode 809 note: two pointers compare runs of equal characters; when the lengths differ, the run in s must be at least 3 and no shorter than the word’s run, and both strings must end together

LeetCode 3823 note: two converging-pointer passes, swapping only letters the first time and only special characters the second

LeetCode 2570 note: merge two id-sorted arrays with two pointers, adding values when ids match

LeetCode 2540 note: one pointer per sorted array, advance the smaller side, and the first match is the answer

LeetCode 581 note: scanning left to right with a running max, the last index below it is the right boundary; scanning right to left with a running min, the last index above it is the left boundary

LeetCode 3555 note: every length-k window is one instance of LeetCode 581; with n only 1000, two O(k) scans per window are enough

LeetCode 1793 note: expand from k toward the larger neighbor each step so the minimum stays as large as possible for each length; at a boundary, only the other side can grow

LeetCode 1989 note: process the “it” players left to right, each catching the leftmost uncaught person in range, with a right-only pointer to the next candidate

LeetCode 2972 note: what remains is a strictly increasing prefix plus suffix; for each valid prefix end, a right-only pointer finds the first suffix start that can follow it, and every later start works too

LeetCode 2122 note: the smallest number is always in lower, so enumerating its partner fixes k; after sorting, two pointers pair each smallest unmatched number with the number 2k above it

LeetCode 658 note: shrink two pointers from both ends to length k, or binary search the window start

LeetCode 633 note: converge a from 0 and b from isqrt(c); shrink b when the sum is too big and grow a when it is too small; watch for 0, a == b, and floating-point square roots