◼ 有效的括号 ：
https://leetcode-cn.com/problems/valid-parentheses/solution/
◼ 括号的分数
https://leetcode-cn.com/problems/score-of-parentheses
◼ 逆波兰表达式求值
https://leetcode-cn.com/problems/evaluate-reverse-polish-notation/
◼ 基本计算器
https://leetcode-cn.com/problems/basic-calculator/comments/
