Leetcode: First Missing Positive

题目

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.

 

思路

1. 排序是 nlogn, 没有其他想法了

2. 参考别人思路. 将 A[i] 月 A[A[i]] 替换, 第二遍 PASS 检查 A[i] == i

 

代码

class Solution {

public:

    int firstMissingPositive(int A[], int n) {

        if(n == 0)

            return 1;

            

		for(int i = 0; i < n; i ++) {

		//	cout << A[i] << endl;

			if(A[i] == i+1 || A[i] <= 0 || A[i] >= n) {

				continue;

			}else {

				swap(A[i], A[A[i]-1]);

				//cout << A[i] << A[A[i]-1] << endl;

				if(A[i] >= 1 && A[i] != A[A[i]-1])

					i--;

			}

		}

		for(int i = 0; i < n; i ++) {

			if(A[i] != i+1)

				return i+1;

		}

		return n+1;

    }

};