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

	private void bucketSort(int[] A) {
		for (int i = 0; i < A.length; i++) {
			while (A[i] != i + 1) {
				if (A[i] > A.length || A[i] < 1 || A[i] == A[A[i] - 1])
					break;
				int temp =A[A[i]-1];
				A[A[i] - 1] = A[i];
				A[i] = temp;
			}
		}
	}
}