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# 0020. Valid Parentheses

<https://leetcode.com/problems/valid-parentheses>

## Description

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 `'()[]{}'`.

## ac

```java
class Solution {
    public boolean isValid(String s) {
        // edge cases
        if (s == null || s.length() <= 1) return false;

        Stack<Character> stack = new Stack<>();
        for (int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);
            if (c == '(' || c == '{' || c == '[') {
                stack.push(c);
            } else {
                if (stack.isEmpty()) return false;
                char p = stack.pop();
                if (c == ')' && p == '(' || c == '}' && p == '{' || c == ']' && p == '[') {
                    continue;
                } else {
                    return false;
                }
            }
        }

        return stack.isEmpty();
    }
}
```