Given an integer array `nums`, return the third distinct maximum number in this array. If the third maximum does not exist, return the maximum number.

Example 1:

    Input: nums = [3,2,1]
    Output: 1
    Explanation:
    The first distinct maximum is 3.
    The second distinct maximum is 2.
    The third distinct maximum is 1.

Example 2:

    Input: nums = [1,2]
    Output: 2
    Explanation:
    The first distinct maximum is 2.
    The second distinct maximum is 1.
    The third distinct maximum does not exist, so the maximum (2) is returned instead.

Example 3:

    Input: nums = [2,2,3,1]
    Output: 1
    Explanation:
    The first distinct maximum is 3.
    The second distinct maximum is 2 (both 2's are counted together since they have the same value).
    The third distinct maximum is 1.

Constraints:

- `1 <= nums.length <= 104`
- `-231 <= nums[i] <= 231 - 1`

Follow up: Can you find an `O(n)` solution?