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) {
   if(A.length<=0){
        return 1;
    }
    for(int i=0;i<A.length;i++){
        if(A[i]>0 && A[i]<=A.length && A[i]!=A[A[i]-1]){
            int temp=A[A[i]-1];
            A[A[i]-1]=A[i];
            A[i]=temp;
            i--;
        }
    }
    for(int j=0;j<A.length;j++){
        if(A[j]!=j+1)
            return j+1;
    }
    
    return A.length+1;  
    }  
}