【leetcode】Valid Parentheses

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

The brackets must close in the correct order, "()" and "()[]{}" are all valid but "(]" and "([)]" are not.

 

題解：典型的棧的應用。遇到"({["中的一種就壓棧，遇到")}]"中的一種就和棧頂元素比較，如果匹配，就繼續迴圈，否則返回false。

代碼如下：

 1 public class Solution { 2     public boolean isEqual(char a,char b){ 3         return (a == ‘(‘ && b ==‘)‘)||(a == ‘[‘ && b ==‘]‘)||(a == ‘{‘ && b == ‘}‘); 4     } 5     public boolean isValid(String s) { 6         if(s == null || s.length() == 0) 7             return true; 8         Stack<Character> stack = new Stack<Character>(); 9         10         for(int i = 0;i < s.length();i++){11             if("({[".contains(String.valueOf(s.charAt(i))))12                 stack.push(s.charAt(i));13             else {14                 if(stack.isEmpty() || !isEqual(stack.peek(),s.charAt(i)))15                     return false;16                 else17                     stack.pop();18             }19         }20         return stack.isEmpty();21     }22 }