# 20.Valid Parentheses

**20.Valid Parentheses**

难度:Easy

> 给定一个只包括 '('，')'，'{'，'}'，'\['，']' 的字符串，判断字符串是否有效。

有效字符串需满足： 左括号必须用相同类型的右括号闭合。 左括号必须以正确的顺序闭合。 注意空字符串可被认为是有效字符串。

```
示例 1:

输入: "()"
输出: true
示例 2:

输入: "()[]{}"
输出: true
示例 3:

输入: "(]"
输出: false
示例 4:

输入: "([)]"
输出: false
示例 5:

输入: "{[]}"
输出: true 
```

方法：使用堆栈判断所有括号即可。代码如下：

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

        }
        if(ss.empty()) return true;
        return false;
    }
};
```


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