414. Third Maximum Number

// LeetCode URL: https://leetcode.com/problems/third-maximum-number/
import java.util.TreeSet;

/**
 * Using 3 variables.
 *
 * Time Complexity: O(N)
 *
 * Space Complexity: O(1)
 */
class Solution1 {
    public int thirdMax(int[] nums) throws IllegalArgumentException {
        if (nums == null || nums.length == 0) {
            throw new IllegalArgumentException("Invalid or Empty input nums array");
        }
        if (nums.length == 1) {
            return nums[0];
        }

        long max1 = Long.MIN_VALUE;
        long max2 = Long.MIN_VALUE;
        long max3 = Long.MIN_VALUE;

        for (int num : nums) {
            if (num == max1 || num == max2 || num == max3) {
                continue;
            }
            if (num > max1) {
                max3 = max2;
                max2 = max1;
                max1 = num;
            } else if (num > max2) {
                max3 = max2;
                max2 = num;
            } else if (num > max3) {
                max3 = num;
            }
        }

        return (int) (max3 == Long.MIN_VALUE ? max1 : max3);
    }
}

/**
 * Using TreeSet
 *
 * Time Complexity: O(N log N)
 *
 * Space Complexity: O(N)
 */
class Solution2 {
    public int thirdMax(int[] nums) throws IllegalArgumentException {
        if (nums == null || nums.length == 0) {
            throw new IllegalArgumentException("Invalid or Empty input nums array");
        }

        TreeSet<Integer> set = new TreeSet<>();
        for (int num : nums) {
            set.add(num);
            if (set.size() > 3) {
                set.pollFirst();
            }
        }

        return set.size() == 3 ? set.first() : set.last();
    }
}