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.

[Thoughts]:感觉有点点无理取闹的一个题。

public class Solution {
    public int firstMissingPositive(int[] A) {
        int len = A.length;
        for(int i=0; i<len; i++){
            while(A[i] != i+1){
                if(A[i]<=0 || A[i]>len || A[i]==A[A[i]-1])
                    break;
                int temp = A[i];
                A[i] = A[temp-1];
                A[temp-1]=temp;
            }
        }
        
        for(int j=0; j<len; j++){
            if(A[j]!=j+1)
                return j+1;
        }
        
        return len+1;
    }
}