# 137. Single Number II
## 1.問題
* 給予一個整數(正, 負)序列, 每個數字重複3次, 找出只有出現一次的數字
## 2.想法
* 提問
* function header, parameter
* test input
* 觀察
* 識別一個數字是否出現兩次可以用XOR, 但第三次時無法歸零, 因此可以想辦法讓第三次時歸0: & \~(上一次的狀態)
* 說明想法
* 測試計算複雜度
## **3.程式碼**
```
class Solution {
public:
int singleNumber(vector& nums) {
int a = 0, b = 0;
for (int i = 0; i < nums.size(); i++) {
a = ((a ^ nums[i]) & ~b);
b = ((b ^ nums[i]) & ~a);
}
return a | b;
}
};
```
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