docs(docs): add Valid Parentheses solution page

apps/docs/content/(string)/20-valid-parentheses.mdx

@@ -0,0 +1,54 @@
+---
+title: '20. Valid Parentheses'
+description: Given a string s containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid
+sidebar:
+  label: 'Valid Parentheses'
+  badge: 'Easy'
+---
+
+<Badge variant="accent">Stack</Badge>
+
+### Example 1:
+- Input: `s = "()"`
+- Output: `true`
+
+### Example 2:
+- Input: `s = "()[]{}"`
+- Output: `true`
+
+### Example 3:
+- Input: `s = "(]"`
+- Output: `false`
+
+### Example 4:
+- Input: `s = "([])"`
+- Output: `true`
+
+### Example 5:
+- Input: `s = "([)]"`
+- Output: `false`
+
+### Constraints:
+
+- `1 <= s.length <= 10^4`
+- `s` consists of parentheses only '()[]{}'.
+
+## Solution
+
+```py
+class Solution:
+    def isValid(self, s: str) -> bool:
+        stack = []
+        pairs = {")": "(", "]": "[", "}": "{"}
+
+        for c in s:
+            if c in pairs:
+                if stack and pairs[c] == stack[-1]:
+                    stack.pop()
+                else:
+                    return False
+            else:
+                stack.append(c)
+
+        return True if not stack else False
+```