Given integer array nums, return the third 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 third maximum is 1. Example 2: Input: nums = [1,2] Output: 2 Explanation: The third maximum does not exist, so the maximum (2) is returned instead. Example 3: Input: nums = [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. Constraints: 1 <= nums.length <= 104 -231 <= nums[i] <= 231 - 1 Follow up: Can you find an O(n) solution?