Single Numbe

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[] nums) {        if (nums == null || nums.length < 0) {            return 0;        }        if (nums.length == 0) {            return nums[0];        }        int flag = nums[0];        for (int i = 1; i < nums.length; i++) {            flag ^= nums[i];//异或        }        return flag;    }}