Here's my solution, use insert sort:

```
class Solution {
public:
int thirdMax(vector<int>& nums) {
int a[3] = {0}, diff = 0;
a[0] = a[1] = a[2] = INT_MIN;
bool contains_min = false;
for (int i = 0; i < nums.size(); i++) {
if (nums[i] == INT_MIN) contains_min = true;
if (a[0] == nums[i] || a[1] == nums[i] || a[2] == nums[i]) continue;
int mi = 0;
if (nums[i] > a[mi]) {
while (nums[i] > a[mi + 1] && mi < 2) {
a[mi] = a[mi + 1];
++mi;
}
a[mi] = nums[i];
}
++diff;
}
if (contains_min) ++diff;
if (diff < 3) return a[2];
else return a[0];
}
};
```

or we can use heap here.