[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.

http://www.cnblogs.com/AnnieKim/archive/2013/04/21/3034631.html

public class Solution {
    public int firstMissingPositive(int[] A) {
        if(A==null||A.length==0)
            return 1;
        int n=A.length;
        for(int i=0;i<n;){
            if((A[i]!=i+1)&&A[i]>=1&&A[i]<=A.length&&A[A[i]-1]!=A[i]){
                int temp=A[A[i]-1];
                A[A[i]-1]=A[i];
                A[i]=temp;
            }else{
                ++i;
            }
        }
        for(int i=0;i<A.length;++i){
            if(A[i]!=i+1)
                return i+1;
        }
        return A.length+1;
    }
}