414. Third Maximum Number

 1 class Solution {
 2 public:
 3     int thirdMax(vector<int>& nums) {
 4         set<int> MaxSet;
 5         for(int i=0; i<nums.size(); i++){
 6             MaxSet.insert(nums[i]);
 7             if(MaxSet.size()>3) MaxSet.erase(MaxSet.begin());
 8         }
 9         if(MaxSet.size()==3) return *MaxSet.begin();
10         else return *MaxSet.rbegin();
11     }
12 };