docs(docs): add 143 Reorder List documentation

apps/docs/content/(02-nodal-grid)/linked-list/143_reorder-list.mdx

@@ -0,0 +1,61 @@
+---
+title: '143. Reorder List'
+description: 'You are given the head of a singly linked-list. The list can be represented as:'
+icon: dot
+topics:
+  - { name: "Linked List", slug: "linked-list" }
+  - { name: "Two Pointers", slug: "two-pointers" }
+  - { name: "Stack", slug: "stack" }
+  - { name: "Recursion", slug: "recursion" }
+issue: "https://github.com/prdlk/leetcode/issues/81"
+sidebar:
+  label: 'Reorder List'
+  badge: 'Medium'
+---
+
+### Example 1:
+- Input: `head = [1,2,3,4]`
+- Output: `[1,4,2,3]`
+
+### Example 2:
+- Input: `head = [1,2,3,4,5]`
+- Output: `[1,5,2,4,3]`
+
+### Constraints:
+
+- The number of nodes in the list is in the range [1, 5 * 10^4].
+- `1 <= Node.val <= 1000`
+
+## Solution
+
+```py
+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, val=0, next=None):
+#         self.val = val
+#         self.next = next
+class Solution:
+    def reorderList(self, head: ListNode | None) -> None:
+        # Find middle
+        slow, fast = head, head.next
+        while fast and fast.next:
+            slow = slow.next
+            fast = fast.next.next
+
+        # Reverse second half
+        second = slow.next
+        prev = slow.next = None
+        while second:
+            tmp = second.next
+            second.next = prev
+            prev = second
+            second = tmp
+
+        # merge two halfs
+        first, second = head, prev
+        while second:
+            tmp1, tmp2 = first.next, second.next
+            first.next = second
+            second.next = tmp1
+            first, second = tmp1, tmp2
+```