leetcode--Single Number

Given an array of integers, every element appears twice except for one. Find that single one.

Note:
Your algorithm should have a linear runtime complexity. Could you implement it without using extra memory?

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public class Solution {
    public int singleNumber(int[] A) {
        int once = 0, len = A.length;
        for(int i = 0; i < len; ++i)
            once ^= A[i];
        return once;
    }
}