feat: refactored docs structure

apps/docs/content/(01-linear-structures)/two-pointers/(problems)/125_valid-palindrome.mdx

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+---
+title: '125. Valid Palindrome'
+description: A phrase is a palindrome if, after converting all uppercase letters into lowercase letters and removing all non-alphanumeric characters, it reads the same forward and backward. Alphanumeric characters include letters and numbers
+icon: dot
+topics:
+  - { name: "Two Pointers", slug: "two-pointers" }
+  - { name: "String", slug: "string" }
+video: "jJXJ16kPFWg"
+issue: "https://github.com/prdlk/leetcode/issues/41"
+sidebar:
+  label: 'Valid Palindrome'
+  badge: 'Easy'
+---
+
+### Example 1:
+- Input: `s = "A man, a plan, a canal: Panama"`
+- Output: `true`
+- Explanation: `"amanaplanacanalpanama"` is a palindrome.
+
+### Example 2:
+- Input: `s = "race a car"`
+- Output: `false`
+- Explanation: `"raceacar"` is not a palindrome.
+
+### Example 3:
+- Input: `s = " "`
+- Output: `true`
+- Explanation: `s` is an empty string `""` after removing non-alphanumeric characters. Since an empty string reads the same forward and backward, it is a palindrome.
+
+### Constraints:
+
+- `1 <= s.length <= 2 * 10^5`
+- `s` consists only of printable ASCII characters.
+
+## Approach
+
+```mermaid
+flowchart TD
+  S(["isPalindrome(s)"]) --> N["normal = s stripped of non-alphanumerics, lowercased"]
+  N --> I["i = 0, j = normal.length - 1"]
+  I --> W{"i < j?"}
+  W -- no --> E(["return true — the pointers met in the middle"])
+  W -- yes --> Q{"normal[i] != normal[j]?"}
+  Q -- yes --> F(["return false"])
+  Q -- no --> A["i++; j-- — step both inward"]
+  A --> W
+```
+
+## Solution
+
+```js
+/**
+ * @param {string} s
+ * @return {boolean}
+ */
+var isPalindrome = function(s) {
+  let normal = s.replace(/[^a-zA-Z0-9]/g, "").toLowerCase()    
+  let i = 0, j = normal.length - 1;
+
+  while (i < j) {
+    if(normal[i] !== normal[j]) {
+      return false;
+    }
+    i++;
+    j--;
+  }
+  return true;
+};
+```