414. Third Maximum Number

Problem:

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.

思路：

Solution (C++):

int thirdMax(vector<int>& nums) {
    if (nums.empty())  return 0;
    int n = nums.size(), count = 0;
    sort(nums.begin(), nums.end(), greater<int>());
    for (int i = 0; i < n; ++i) {
        while (i < n-1 && nums[i] == nums[i+1])  ++i;
        ++count;
        if (count == 3)  return nums[i];  
    }
    if (count < 3)  return nums[0];
    return 0;
}

性能：

Runtime: 20 ms  Memory Usage: 9.6 MB

思路：

Solution (C++):

性能：

Runtime: ms  Memory Usage: MB