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) {
        if (A.length == 0) {
            return 1;
        }
        int i=0;
        int n = A.length;
        while (i < n) {
            if (A[i] != i + 1 && A[i] >= 1 && A[i] <= n && A[A[i] - 1] != A[i]) {
                swap(A,i,A[i]-1);
            }else ++i;
        }
        for (int j = 0; j < A.length; j++) {
            if (A[j] != j + 1) {
                return j + 1;
            }
        }
        return n + 1;
    }
        void swap(int[] A, int a, int b) {
        int temp = A[a];
        A[a] = A[b];
        A[b] = temp;
    }
}