【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.

找到第一个没有出现的正整数

 

思路：

http://tech-wonderland.net/blog/leetcode-first-missing-positive.html

就是把出现的正整数a都放在a-1的位置，然后从头开始历遍，发现A[i] != i + 1的情况就是所要的结果

 

代码：

class Solution {
public:
    int firstMissingPositive(int A[], int n) {
        int i = 0, j;
        while (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 (j = 0; j < n; j++)
            if (A[j] != j + 1)
                return j + 1;
        return n + 1;
    }
};