Example 1:
Input: [3, 2, 1]
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Output: 1
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Explanation: The third maximum is 1.
Example 2:
Input: [1, 2]
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Output: 2
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Explanation: The third maximum does not exist, so the maximum (2) is returned instead.
Example 3:
Input: [2, 2, 3, 1]
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Output: 1
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Explanation: Note that the third maximum here means the third maximum distinct number.
Both numbers with value 2 are both considered as second maximum.

class Solution {
public:
    int thirdMax(vector<int>& nums) {
        if(nums.size()<3) return *max_element(nums.begin(),nums.end());
        priority_queue<int,vector<int>,greater<int>>three;
        unordered_set<int>ele;
​
        for(int i=0;i<nums.size();i++)
        {
            if(ele.count(nums[i])) continue;
            three.push(nums[i]);
            ele.insert(nums[i]);
        }
        if(three.size()==1) return three.top();
        if(three.size()==2)    {three.pop();    return three.top();}
        while(three.size() !=3) three.pop();
        return three.top();
​
    }
};