1575 articles
LeetCode 66 looks trivial until you hit [9,9,9]. Learn why interviewers love this problem, how carry propagation cascades through digits, the critical all-9s edge case that requires a new array, and why early termination makes your solution elegant — with Python and JavaScript solutions fully explained.
Most candidates try to merge nums1 and nums2 from the front and run into a subtle overwrite bug. Learn why starting from the back is the key insight, how three pointers keep the logic clean, and how this same trick powers the merge step in Merge Sort. Full Python and JavaScript solutions included.
Master the write pointer pattern — the canonical technique for in-place array modification. Full walkthrough of LeetCode 26 with visual dry run, common mistakes, and the LC 80 generalization. Python and JavaScript solutions included.
Find the one element that appears once while every other appears twice. The XOR bit trick delivers O(n) time and O(1) space — no extra memory, no sorting. Master the three XOR properties that make it work, then see how interviewers escalate to Single Number II and III.
Two problems, one pair of concepts: LC 349 asks for the unique intersection (HashSet), LC 350 asks for the frequency-aware intersection (HashMap). Master all three approaches for LC 349 — HashSet, sort+two pointers, binary search — then learn why the follow-up questions on LC 350 are what Google and Amazon actually care about: sorted input, skewed sizes, and data that does not fit in memory.