LeetCode：Valid Parentheses

题目链接：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.

显然是一个出栈入栈匹配的问题，并且不是完整的四则运算式子，仅有大中小三种括号，不难。

代码如下：

public class Solution {
    public boolean isValid(String s) {
        int length = s.length();
        Stack<Character> stack = new Stack<>();
        for (int i=0; i<length; i++) {
            char c = s.charAt(i);
            if (c == '(' || c == '[' || c == '{') {
                stack.push(c);
            } else if (stack.isEmpty()){
                return false;
            } else {
                char top = stack.peek();
                if ((c == ')' && top == '(')
                        || (c == ']' && top == '[')
                        || (c == '}' && top == '{')) {
                    stack.pop();
                } else {
                    return false;
                }
            }
        }
        return stack.isEmpty();
    }
}