# LeetCode 20. Valid Parentheses

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

An input string is valid if:

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

**Example 1:**

```
Input: s = "()"
Output: true
```

**Example 2:**

```
Input: s = "()[]{}"
Output: true
```

**Example 3:**

```
Input: s = "(]"
Output: false
```

**Example 4:**

```
Input: s = "([)]"
Output: false
```

**Example 5:**

```
Input: s = "{[]}"
Output: true
```

**Constraints:**

* `1 <= s.length <= 104`
* `s` consists of parentheses only `'()[]{}'`.

## Solution:

[English Version in Youtube](https://youtu.be/rh4T1sI8krU)

[中文版解答Youtube Link](https://youtu.be/oAu0zWo3fNw)

[中文版解答Bilibili Link](https://www.bilibili.com/video/BV1Ty4y1e7dQ/)

```
class Solution {
public:
    bool isValid(string s) {
        stack<char> container;
        for (char c : s) {
            if (container.empty()) {
                container.push(c);
            } else {
                if (c == '(' || c == '{' || c == '[') {
                    container.push(c);
                } else {
                    char v = container.top();
                    if ( (c == ')' && v == '(') || (c == '}' && v == '{') || (c == ']' && v == '[')) {
                        container.pop();
                    } else {
                        return false;
                    }
                }
            }
        }
        return container.empty();
    }
    
};
```


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