[LeetCode]First Missing Positive

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.

方法：每次判断a[i]==i。

if true，i++

else 将交换a[i]和a[a[i]]。让a[a[i]] = a[i];

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