Cyclic-sort

First Missing Positive — Cyclic Sort Index Marking [LC 41]

LeetCode 41 — asked at Amazon, Google, and Microsoft. Find the smallest missing positive integer in O(n) time and O(1) space. Use the array itself as a hash map: place each number i at index i-1, then scan for the first mismatch. The answer must be in [1, n+1].