136 single number

136. Single Number¶

Given a non-empty 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?

Example 1:

Input: [2,2,1]
Output: 1

Example 2:

Input: [4,1,2,1,2]
Output: 4

这道题目涉及位操作。^(xor, 异或): 当两个位不同时，输出true/1。异或的真值表：

所以对于整数来说：

0 ^ 0 = 0 
n ^ 0 = n 
n ^ n = 0

如果把题目中的所有整数接连异或，由于出现两次的数字异或结果为0，最后剩下的就是出现一次的数字。

Java

public int singleNumber(int[] nums) {
    int res = 0;
    for (int num : nums)
        res ^= num;
    return res; 
}

Python

def singleNumber(self, nums):
    """
    :type nums: List[int]
    :rtype: int
    """
    n = 0
    for num in nums:
        n = n^num
    return n