LeetCode - Single Number

https://leetcode.com/problems/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?

几种方法：

1. brute force，每遇到一个数都遍历数组剩下的数，时间复杂度O(n^2)

2. hashtable，时间复杂度O(n), 空间复杂度O(n)

3. 排序，时间复杂度O(n*lgn)，空间复杂度O(1)

3. 异或，两个相同的数异或就是0，任何数异或0都不变，所以异或到最后剩下的数就是只出现了一次的数。时间复杂度O(n)，空间复杂度O(1)

public class Solution {
    public int singleNumber(int[] A) {
        int ans = A[0];
        for(int i=1; i<A.length; i++){
            ans ^= A[i];
        }
        return ans;
    }
}