Use an array to count every number. Then, return the number with two and zero occurrence.

```
class Solution {
public:
vector<int> findErrorNums(vector<int>& nums) {
int cnt[nums.size()] = {0};
for(int n : nums) cnt[n-1]++;
int a = 0, b = 0;
for(int i = 0; i < nums.size(); i++){
if(cnt[i] == 2) a = i+1;
if(cnt[i] == 0) b = i+1;
}
return {a,b};
}
};
``
```