LeetCode: Valid Parentheses 解题报告

Valid Parentheses

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.

SOLUTION1:

使用stack来解决的简单题目。所有的字符依次入栈

1. 遇到成对的括号弹栈，弹栈不成对返回false.

2. 栈为空只能压入左括号

3. 扫描完成时，栈应该为空，否则返回FALSE.

SOLUTION 2:

使用堆栈解决，比较简单。push右括号时，检查左括号在不在，如果不在，返回false,否则弹出一个左括号。

最后看栈是否为空，不为空代表括号数不对称，也要返回false;

 public class Solution {
     public boolean isValid(String s) {
         if (s == null) {
             return false;
         }

         int len = s.length();

         // bug 1: don't use s as the name of the stack.
         Stack<Character> stk = new Stack<Character>();

         for (int i = 0; i < len; i++) {
             char c = s.charAt(i);
             switch(c) {
                 case '(':
                 case '[':
                 case '{':
                     stk.push(c);
                     break;
                 case ')':
                     if (!stk.isEmpty() && stk.peek() == '(') {
                         stk.pop();
                     } else {
                         return false;
                     }
                     break;
                 case '}':
                     if (!stk.isEmpty() && stk.peek() == '{') {
                         stk.pop();
                     } else {
                         return false;
                     }
                     break;
                 case ']':
                     if (!stk.isEmpty() && stk.peek() == '[') {
                         stk.pop();
                     } else {
                         return false;
                     }
                     break;
             }
         }

         return stk.isEmpty();
     }
 }

https://github.com/yuzhangcmu/LeetCode_algorithm/blob/master/stack/IsValid.java