LeetCode：Valid Parentheses

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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();    }}

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