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.

 

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