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;
    }
}