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.

 

解决：

正负来来记录，值映射到位置。

 

public class Solution {      public int firstMissingPositive(int[] A) {        for (int i=0; i < A.length; i++){            if (A[i]<=0) A[i]=(A.length+2);//把0与负mo去        }                for (int i=0;i<A.length;i++){            if (Math.abs(A[i])<A.length+1){//把0与负mo去                int cur = Math.abs(A[i])-1;                A[cur] = -Math.abs(A[cur]);//用正负来记录，值映射到位置            }        }                for (int i=0; i < A.length; i++)            if (A[i]>0) return i+1;//没有映射到的位置就是缺的数        return A.length+1;    }}