LeetCode 41: O(n) In-Place Solution for First Missing Positive Integer

LeetCode problem 41 asks for the smallest missing positive integer from an unsorted array. The approach works in three phases: first, out-of-range values are replaced with n+1 to neutralize them. In the second phase, the algorithm uses index-based negative marking to track which positive integers are present. Finally, the first index still holding a positive value reveals the missing integer, or n+1 is returned if none is found. The solution achieves O(n) time complexity without extra space.