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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.
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.
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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;
1 public class Solution { 2 public boolean isValid(String s) { 3 if (s == null) { 4 return false; 5 } 6 7 int len = s.length(); 8 9 // bug 1: don‘t use s as the name of the stack. 10 Stack<Character> stk = new Stack<Character>(); 11 12 for (int i = 0; i < len; i++) { 13 char c = s.charAt(i); 14 switch(c) { 15 case ‘(‘: 16 case ‘[‘: 17 case ‘{‘: 18 stk.push(c); 19 break; 20 case ‘)‘: 21 if (!stk.isEmpty() && stk.peek() == ‘(‘) { 22 stk.pop(); 23 } else { 24 return false; 25 } 26 break; 27 case ‘}‘: 28 if (!stk.isEmpty() && stk.peek() == ‘{‘) { 29 stk.pop(); 30 } else { 31 return false; 32 } 33 break; 34 case ‘]‘: 35 if (!stk.isEmpty() && stk.peek() == ‘[‘) { 36 stk.pop(); 37 } else { 38 return false; 39 } 40 break; 41 } 42 } 43 44 return stk.isEmpty(); 45 } 46 }
https://github.com/yuzhangcmu/LeetCode_algorithm/blob/master/stack/IsValid.java
LeetCode: Valid Parentheses 解题报告
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原文地址：http://www.cnblogs.com/yuzhangcmu/p/4202351.html
