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.

public class Solution{    
    public int firstMissingPositive(int[] A){
        int result = 1; //if the array is empty, we result 1
        if(A.length > 0){
            for(int i = 0; i < A.length; ++i){
                //do not forget the condition A[A[i] - 1] != A[i];
                //otherwise, there is a dead loop on input, for example, [2,2]
                if(A[i] > 0 && A[i] <= A.length && A[i] != i + 1 && A[A[i] - 1] != A[i]){
                    int temp = A[i];
                    int temp1 = A[A[i] - 1];
                    A[A[i] - 1] = A[i];
                    A[i] = temp1;
                    i = (temp > i + 1) ? i - 1 : i;
                }
            }
            for(int i = 0; i < A.length; ++i){
                if(A[i] != i + 1){
                    result = i + 1;
                    break;
                }
                else
                    result = A.length + 1;
            }
        }
        return result;
    }
}