[Leetcode] First Missing Positive

First Missing Positive Mar 8 '12 5401 / 16835

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.

这题目都不能一次过，郁闷

class Solution {
public:
    int firstMissingPositive(int A[], int n) {
        int *a = A, tmp;
        for (int i = 0; i < n; i++) {
            if (a[i] == i+1) continue;
            while (a[i] != i+1) {
                if (a[i] > n || a[i] <= 0) break;
                if (a[a[i]-1] == a[i]) break;
                swap(a[i], a[a[i]-1]);
            }
        }
        for (int i = 0; i < n; ++i)
        {
            if (a[i] != i+1) return i+1;
        }
        return n+1;
    }
};