leetcode刷题笔记二十 有效的括号 Scala版本

leetcode刷题笔记二十 有效的括号 Scala版本

源地址：20. 有效的括号

问题描述：

Given a string containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.

An input string is valid if:

1. Open brackets must be closed by the same type of brackets.
2. Open brackets must be closed in the correct order.

Note that an empty string is also considered valid.

Example 1:

Input: "()"
Output: true

Example 2:

Input: "()[]{}"
Output: true

Example 3:

Input: "(]"
Output: false

Example 4:

Input: "([)]"
Output: false

Example 5:

Input: "{[]}"
Output: true

代码补充：

//本题是一个典型的栈的应用，遇到左侧括号则入栈，遇到右侧括号有两种可能性，栈空或者出栈的括号与右侧入栈括号未能对应返回false
import scala.collection.mutable.Stack
object Solution {
    def isValid(s: String): Boolean = {
        if ( s == "") return true
        val stack = Stack[Char]()
        for (elem <- s){
            elem match {
                case '(' => stack.push(elem)
                case '[' => stack.push(elem)
                case '{' => stack.push(elem)
                case ')' => if (stack.length == 0 || stack.pop != '(') return false
                case ']' => if (stack.length == 0 || stack.pop != '[') return false
                case '}' => if (stack.length == 0 || stack.pop != '{'  ) return false
                case _ => return false
            }
        } 
        //如果左侧括号入栈仍有剩余未匹配，则false
        if (stack.length == 0) return true
        else return false
        
    }
}