20. Valid Parentheses

一,题目

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.

二,思路

1,遇到([{入栈
2,每次遇到) [ }判断栈顶是否能与其匹配,如果可以,栈顶出栈,否则返回true
3,遍历完毕如果栈不为空返回false,否则true

三,代码

public  boolean isValid(String s) {
		if(s == null || s.length() == 0){
			return true;
		}
		
		int len = s.length();
		if(len %2 != 0){
			return false;
		}
		
		Stack<Character> stack = new Stack<Character>();
		for(int i=0; i < len; i++){
			char ch = s.charAt(i);
			if('(' == ch){
				stack.push(ch);
			}else if('[' == ch){
				stack.push(ch);
			}else if('{' == ch){
				stack.push(ch);
			}else if(')' == ch){
				if(stack.size() > 0 && stack.peek() == '('){
					stack.pop();
				}else{
					return false;
				}
			}else if(']' == ch){
				if(stack.size() > 0 && stack.peek() == '['){
					stack.pop();
				}else{
					return false;
				}
			}else if('}' == ch){
				if(stack.size() > 0 && stack.peek() == '{'){
					stack.pop();
				}else{
					return false;
				}
			}
		}
		return stack.isEmpty();
	}