two pointers

Sliding window is one of the highest-yield coding patterns, and this is its canonical problem. The real signal is recognizing when a window collapses an O(n squared) scan into one linear pass, and maintaining the invariant cleanly.

A classic that rewards the two-pointer insight over brute force. The signal is realizing water at each position is bounded by the min of the max walls on each side, then computing it in one pass. Here is the answer.

Minimum Window Substring is the hard sliding-window problem that tests expand-and-contract with a character-count map. The signal is the grow-to-valid then shrink-to-minimal pattern, checked in O(1) per step.

Sorting an array of three values in one pass tests the three-pointer partition (Dutch National Flag). The signal is the low/mid/high invariant and knowing when not to advance mid. Here is the answer.

This classic has a clean O(n^2) expand-around-center solution that beats the naive O(n^3). The signal is expanding from every center and handling odd and even lengths separately. Here is the answer.

A row- and column-sorted matrix rewards one observation: start from a corner and kill an entire row or column each step. That turns an O(m*n) scan into O(m+n), and the choice of corner is not arbitrary.

Floyd's tortoise and hare detects a cycle in O(1) space, but the part that separates strong candidates is the second phase: a short distance argument that pinpoints exactly where the loop begins. Here is the answer and the math.

Two lists that share a tail must intersect at a single node, but matching their lengths without counting is the elegant move interviewers want. Here is the two-pointer trick that makes the lengths cancel out, plus the answer.

The two-pass length-then-delete solution works, but the interview wants a single pass: a gap of N between two pointers, plus a dummy head that makes deleting the real head fall out for free. Here is the clean answer.

The naive triple loop is O(n cubed). Sort first, then fix one element and converge two pointers for O(n squared). The real test is deduplication: returning unique triplets without a set. Here is the clean template.