414. Third Maximum Number

Given a non-empty array of integers, return the third maximum number in this array. If it does not exist, return the maximum number. The time complexity must be in O(n).

Example 1:

Input: [3, 2, 1]

Output: 1

Explanation: The third maximum is 1.

Example 2:

Input: [1, 2]

Output: 2

Explanation: The third maximum does not exist, so the maximum (2) is returned instead.

Example 3:

Input: [2, 2, 3, 1]

Output: 1

Explanation: Note that the third maximum here means the third maximum distinct number.
Both numbers with value 2 are both considered as second maximum.

求第3大的数，数组中有重复数字

C++（6ms）：

class Solution {
public:
    int thirdMax(vector<int>& nums) {
        set<int> res ;
        for(int i : nums){
            res.insert(i) ;
            if (res.size() > 3)
                res.erase(res.begin()) ;
        }
        return res.size()==3?*res.begin():*res.rbegin() ;
    }
};