Two-Pointers

LeetCode 541 note: in every block of 2k reverse the first k; when fewer than k remain reverse them all, which slicing handles automatically

LeetCode 455 note: sort children and cookies, satisfying the least greedy child with the smallest cookie that suffices; each pointer walks once

LeetCode 392 note: two pointers; the pointer on t always advances, and the one on s advances on a match; bound-check both

LeetCode 350 note: an intersection keeping multiplicity, where Counter’s & takes the smaller count from each side

LeetCode 349 note: convert both arrays to sets and intersect them

LeetCode 345 note: the same two pointers as LC 917, with the condition switched to “is a vowel” (both cases)

LeetCode 344 note: swap with two pointers and move inward to reverse in place

LeetCode 283 note: the write pointer advances only on non-zero values, swapping with them, which pushes zeros to the back

LeetCode 234 note: find the middle with fast and slow pointers, reverse the second half, and compare from both ends with O(1) extra space

LeetCode 202 note: repeatedly take the sum of squared digits, which reaches 1 or enters a cycle; detect it with a set of seen numbers or fast and slow pointers

LeetCode 160 note: each pointer walks its own list then the other’s; both cover lenA + lenB steps and reach the intersection, or None, together

LeetCode 141 note: Floyd’s cycle detection, fast two steps and slow one; with a cycle they must meet, each advancing from its own position