Description
Given a string containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.
Example
The brackets must close in the correct order, "()" and "()[]{}" are all valid but "(]" and "([)]" are not.
Challenge
O(n)的时间，n为括号的个数
解题：括号匹配问题，用栈来做较为简单，代码如下：
public class Solution {
/**
* @param s: A string
* @return: whether the string is a valid parentheses
*/
public boolean isValidParentheses(String s) {
// write your code here
Stack<Character>stack = new Stack<Character>();
for(int i = 0; i < s.length(); i++){
if(s.charAt(i) == '(' || s.charAt(i) == '{' || s.charAt(i) == '['){
stack.push(s.charAt(i));
}else{ //pop前要先检查栈是不是空的
if(!stack.isEmpty()){
switch(s.charAt(i)){
case ')':
if(!stack.pop().equals('(')){
return false;
}
break;
case ']':
if(!stack.pop().equals('[')){
return false;
}
break;
case '}':
if(!stack.pop().equals('{')){
return false;
}
break;
}
}else{
return false;
}
}
}
if(stack.isEmpty()){
return true;
}else{
return false;
}
}
}