由于是整数数组。阵列中的每个编号重复的次数为奇数，但是只有1号被重复的偶数次。找到这个数字。

Given is an array of integers. Each number in the array repeats an ODD number of times, but only 1 number is repeated for an EVEN number of times. Find that number.

我在想一个哈希表，每个元素的数量。它需要O（n）的空间。有没有更好的办法？

I was thinking a hash map, with each element's count. It requires O(n) space. Is there a better way?

推荐答案

哈希地图是好的，但你需要存储的每个元素的数模2。 所有这些都将最终被1（奇）除0（偶数）-count元素。

Hash-map is fine, but all you need to store is each element's count modulo 2. All of those will end up being 1 (odd) except for the 0 (even) -count element.

（如阿莱克斯摹说，你并不需要使用运算（数+％2 ），只有XOR（计数^ =为0x1 ）;但任何编译器将优化反正）

(As Aleks G says you don't need to use arithmetic (count++ %2), only xor (count ^= 0x1); although any compiler will optimize that anyway.)