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.

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

        for (int i=0; i<n; i++) {
                if (A[i] != i+1)
                        return i+1;
        }

        return n+1;
    }
};

2. 略过那些不存在合法位置的值，比如负数，或者超过n的数

3.持续交换，直到目标的位置已经存在目标值。或者第2种情况。

First Missing Positive -- leetcode