Add mergeKLists function to merge k sorted lists

23_merge_k_sorted_lists.py

@@ -0,0 +1,73 @@
+"""
+LeetCode Problem #23 — Merge k Sorted Lists (Hard)
+-------------------------------------------------
+You are given an array of k linked-lists, each of which is sorted in ascending order.
+Merge all the linked lists into one sorted linked list and return it.
+
+Example:
+Input: lists = [[1,4,5],[1,3,4],[2,6]]
+Output: [1,1,2,3,4,4,5,6]
+
+"""
+
+import heapq
+
+# Definition for singly-linked list.
+class ListNode:
+    def __init__(self, val=0, next=None):
+        self.val = val
+        self.next = next
+
+class Solution:
+    def mergeKLists(self, lists):
+        # Min-heap to store (node value, list index, node reference)
+        min_heap = []
+
+        # Initialize the heap with the first node of each list
+        for i, node in enumerate(lists):
+            if node:
+                heapq.heappush(min_heap, (node.val, i, node))
+
+        dummy = ListNode(0)
+        current = dummy
+
+        while min_heap:
+            # Extract the smallest node
+            val, i, node = heapq.heappop(min_heap)
+            current.next = node
+            current = current.next
+
+            # If there are more nodes in this list, push the next node into the heap
+            if node.next:
+                heapq.heappush(min_heap, (node.next.val, i, node.next))
+
+        return dummy.next
+
+
+# Example test
+# Helper function to build and print linked lists
+def build_linked_list(values):
+    dummy = ListNode(0)
+    curr = dummy
+    for val in values:
+        curr.next = ListNode(val)
+        curr = curr.next
+    return dummy.next
+
+def print_linked_list(node):
+    res = []
+    while node:
+        res.append(node.val)
+        node = node.next
+    return res
+
+# Example usage
+if __name__ == "__main__":
+    lists = [
+        build_linked_list([1,4,5]),
+        build_linked_list([1,3,4]),
+        build_linked_list([2,6])
+    ]
+
+    result = Solution().mergeKLists(lists)
+    print("Merged Linked List:", print_linked_list(result))