Single Number

Given 2*n + 1 numbers, every numbers occurs twice except one, find it.

Have you met this question in a real interview? Yes
Example
Given [1,2,2,1,3,4,3], return 4

Challenge
One-pass, constant extra space.

public class Solution {    /**     *@param A : an integer array     *return : a integer      */    public int singleNumber(int[] A) {        if (A.length == 0) {            return 0;        }        int n = A[0];        for(int i = 1; i < A.length; i++) {            n = n ^ A[i];        }        return n;    }}