Adding my notes + solution for 2181

Author: jeremymanning

problems/2181/jeremy.md

@@ -0,0 +1,55 @@
+# [Problem 2181: Merge Nodes in Between Zeros](https://leetcode.com/problems/merge-nodes-in-between-zeros/description/)
+
+## Initial thoughts (stream-of-consciousness)
+- This seems pretty straightforward...
+- I think we can just keep a running sum of the nodes since the previous zero
+- Then once we hit another zero, set the value of the current node to the sum
+- If there are no more nodes, return the head node
+- Any edge cases to consider?
+  - If there are two 0s in a row, what should we do?  It looks like that case is not allowed.
+  - We also don't need to account for lists that don't have any closing 0s, since the list length must be greater than or equal to 3 and the first and last values are guaranteed to be 0.
+
+## Refining the problem
+- I think we can solve this in $O(n)$ time (where $n$ is the number of nodes in the original linked list)
+
+## Attempted solution(s)
+
+```python
+class ListNode:
+    def __init__(self, val=0, next=None):
+        self.val = val
+        self.next = next
+
+class Solution:
+    def mergeNodes(self, head: Optional[ListNode]) -> Optional[ListNode]:
+      x = 0
+      new_head = ListNode()
+      current = new_head
+      
+      n = head.next
+      while n.next is not None:
+          if n.val == 0:
+              current.val = x
+              next_node = ListNode()
+              current.next = next_node
+              current = current.next
+              x = 0
+          else:
+              x += n.val
+          
+          n = n.next
+
+      current.val = x
+      return new_head
+```
+
+- Both given test cases pass
+- Trying a new test case: `head = [0, 1, 0, 2, 0, 1, 2, 3, 4, 5, 0, 1, 0, 10, 1000, 1000, 1000, 0, 10, 10, 10, 10, 0]` (also passes)
+- Submitting...
+  
+![Screenshot 2024-07-03 at 10 51 14 PM](https://github.com/ContextLab/leetcode-solutions/assets/9030494/ce0a50a6-9a97-4a5f-aeb6-68928f49d372)
+
+Solved!
+
+
+