# 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(vector<int>& nums) {
long first = LONG_MIN, second = LONG_MIN, third = LONG_MIN;
for (int num : nums) {
if (num > first) {
third = second;
second = first;
first = num;
} else if (num > second && num < first) {
third = second;
second = num;
} else if (num > third && num < second) {
third = num;
}
}
return (third == LONG_MIN || third == second) ? first : third;
}
};
``````

``````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.begin() : *s.rbegin();
}
};
``````

https://discuss.leetcode.com/topic/63903/short-easy-c-using-set

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