FirstMissingPositive.h

/*
 Author:     Annie Kim, anniekim.pku@gmail.com
 Date:       Apr 21, 2013
 Problem:    First Missing Positive
 Difficulty: Medium
 Source:     http://leetcode.com/onlinejudge#question_41
 Notes:
 Given an unsorted integer array, find the first missing positive integer.
 For example,
 Given [1,2,0] return 3,
 and [3,4,-1,1] return 2.
 Your algorithm should run in O(n) time and uses constant space.

 Solution: Although we can only use constant space, we can still exchange elements within input A!
           Swap elements in A and try to make all the elements in A satisfy: A[i] == i + 1.
           Pick out the first one that does not satisfy A[i] == i + 1.
 */

class Solution {
public:
    int firstMissingPositive(int A[], int n) {
        int i = 0;
        while (i < n)
        {
            if (A[i] != (i+1) && A[i] >= 1 && A[i] <= n && A[A[i]-1] != A[i])
                swap(A[i], A[A[i]-1]);
            else
                i++;
        }
        for (i = 0; i < n; ++i)
            if (A[i] != (i+1))
                return i+1;
        return n+1;
    }
};