strings

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 warm-up that screens for the stack instinct. The signal is recognizing last-opened-first-closed maps to a stack, and handling the edge cases (leftover opens, early close). Here is the pattern.

Edit distance is the canonical 2D string DP, behind spell-check, diff, and fuzzy matching. The signal is defining the subproblem and the insert/delete/replace recurrence. Here is the answer.

Word Break is the classic 1D string DP that trips people who reach for greedy or naive recursion. The signal is 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. The signal is reconstructing the match/mismatch recurrence cold and not confusing subsequence (gaps allowed) with substring (contiguous). 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.

Group Anagrams tests the 'compute a canonical key and bucket by it' pattern. The signal is choosing a key that anagrams share without comparing every pair. Here is the answer.

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

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

Evaluating arithmetic with precedence and nested parentheses is a parsing problem, and a stack handles both cleanly. The traps are operator precedence, multi-digit numbers, and the sign of truncated division. Here is a single-pass solution.