Valid Palindrome II

LeetCode 680 / 1216. Two-pointer with a single allowed deletion (or K deletions in the harder follow-up). Meta-canon phone-screen problem. Requirements Return whether the string is a palindrome after deleting at most 1 character. Follow-up: at most K deletions (LeetCode 1216, hard); recursive two-pointer with memo or interval DP. Examples 'abca' → true (drop b or c); 'abcdef' → false. Notes Two-pointer skip-once is the clean solution: on mismatch, try skipping left or right and recheck. The K-delete extension is the differentiator. Recurse on the next mismatch with k-1, memoize on (l, r, k). Preparation Write the K=1 version in <5 min, then immediately extend to K with memoization on a whiteboard. Drill LeetCode 5 (Longest Palindromic Substring) as a related warm-up — different algorithm, but conditions you to think in palindrome invariants.