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.

class Solution {
    public int thirdMax(int[] A) {
        Set<Integer> set = new TreeSet();
        Queue<Integer> q = new PriorityQueue();
        for(int i: A){
            if(set.add(i)){
                q.offer(i);
            }
            if(q.size() > 3) q.poll();
        }
        if(q.size() == 2) q.poll();
        return q.peek();
    }
}