codes from Leetcode
THE GOAL IS THAT THE NUMBER OF QUESTIONS SOLVED BY MYSELF IN LEETCODE REACH 100. THE GOAL IS THAT THE NUMBER OF QUESTIONS SOLVED BY MYSELF IN LEETCODE REACH 200. THE GOAL IS THAT THE NUMBER OF QUESTIONS SOLVED BY MYSELF IN LEETCODE REACH 300. THE GOAL IS THAT THE NUMBER OF QUESTIONS SOLVED BY MYSELF IN LEETCODE REACH 400. THE GOAL IS THAT THE NUMBER OF QUESTIONS SOLVED BY MYSELF IN LEETCODE REACH 500.