Leetcode20：Valid Parentheses

题目描述

Given a string containing just the characters ‘(’, ‘)’, ‘{’, ‘}’, ‘[’ and ‘]’, determine if the input string is valid.

An input string is valid if:

Open brackets must be closed by the same type of brackets.
Open brackets must be closed in the correct order.

Note that an empty string is also considered valid.

Example 1:

Input: "()"
Output: true

Example 2:

Input: "()[]{}"
Output: true

Example 3:

Input: "(]"
Output: false

Example 4:

Input: "([)]"
Output: false

Example 5:

Input: "{[]}"
Output: true

分析

这道题很容易想到使用栈。如果当前符号是左括号，就压入栈中；如果是右括号，就将栈顶元素弹出，判断是否匹配。最后遍历完字符串的字符后，判断一下栈是否为空即可。

AC代码如下：

class Solution {
public:
    bool isValid(string s) {
        if(s.empty())
        {
        	return true;
		}
		stack<char> temp;
		int length = s.length();
		for(int i = 0; i < length; ++i)
		{
			if(s[i] == ')')
			{
				if(!temp.empty() && temp.top() == '(')
				{
					temp.pop();
					continue;
				}
				else
				{
					return false;
				}
			}
			if(s[i] == ']')
			{
				if(!temp.empty() && temp.top() == '[')
				{
					temp.pop();
					continue;
				}
				else
				{
					return false;
				}
			}
			if(s[i] == '}')
			{
				if(!temp.empty() && temp.top() == '{')
				{
					temp.pop();
					continue;
				}
				else
				{
					return false;
				}
			}
			temp.push(s[i]);
		}
		return temp.empty();
    }
};