https://leetcode.com/problems/third-maximum-number/description/

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.

Solution: Set

Time complexity: O(n)

Space complexity: O(1)

C++

// Author: Huahua
// Running time: 9 ms
class Solution {
public:
  int thirdMax(vector<int>& nums) {
    set<int> s;
    for (int num : nums) {
      s.insert(num);
      if (s.size() > 3) s.erase(s.begin());
    }
    return s.size() != 3 ? *s.rbegin() : *s.begin();
  }
};

Python3

"""
Author: Huahua
Running time: 52 ms
"""
class Solution:
  def thirdMax(self, nums):
    s = set()
    for num in nums:
      s.add(num)
      if len(s) > 3: s.remove(min(s))
    return min(s) if len(s) == 3 else max(s)

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