# 136. Single Number
## 1.問題
* 給予一個整數(正, 負)序列, 每個數字重複2次, 找出只有出現一次的數字
## 2.想法
* 提問: 是否會包括負數？
* function header, parameter
* test input
* 觀察
* 由於最多出現兩次, 應該要想到用XOR, 當同一個bit的值皆為1或0時, 會設為0, 也就是當該bit被設兩次時, 數值會為0
* 說明想法
* 測試計算複雜度
## **3.程式碼**
```
class Solution {
public:
int singleNumber(vector& nums) {
if (nums.empty()) {
return 0;
}
int res = 0;
for (int i = 0; i < nums.size(); i++) {
res ^= nums[i];
}
return res;
}
};
```
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