strings

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

A warm-up that tests for the stack instinct. Interviewers look for recognizing that last-opened-first-closed maps onto a stack, plus handling the edge cases (leftover opens, early close). The pattern follows.

Edit distance is the archetypal 2D string DP, powering spell-check, diff, and fuzzy matching. What interviewers watch for: naming the subproblem and the insert/delete/replace recurrence. Here is the answer.

Word Break is the classic 1D string DP that snags people who reach for greedy or naive recursion. What interviewers watch for: the dp[i] = 'is the prefix of length i segmentable' recurrence, plus why a locally valid split can doom the rest. Here is the answer.

LCS is the template 2D-sequence DP behind diff tools and bioinformatics. Interviewers look for rebuilding the match/mismatch recurrence from memory and not mixing up subsequence (gaps allowed) with substring (contiguous). The answer follows.

Minimum Window Substring is the hard sliding-window problem that probes expand-and-contract with a character-count map. Interviewers look for the grow-to-valid then shrink-to-minimal pattern, verified in O(1) per step.

Group Anagrams probes the 'compute a canonical key and bucket by it' pattern. Interviewers look for choosing a key that anagrams share without comparing every pair. The answer follows.

Decode Ways is a 1D DP whose difficulty sits entirely in the edge cases: zeros and the valid 1-26 range. Interviewers look for the take-one-digit-or-two recurrence plus disciplined validity checks. The answer follows.

This classic has a tidy O(n^2) expand-around-center solution that outdoes the naive O(n^3). Interviewers look for expanding from every center and handling odd and even lengths separately. The answer follows.

Generating then filtering costs O(2^(2n)) and wastes effort. Backtracking with two counters builds only valid strings by applying the balance rule at every step. Here is the template and why the pruning conditions are exactly right.

Working out arithmetic with precedence and nested parentheses is a parsing task, and a single stack manages both neatly. The traps are operator precedence, multi-digit numbers, and the sign of truncated division. Here is a one-pass solution.