[leetcode] First Missing Positive

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.

就地调整数组的顺序，然后注意边界顺序。

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