add 23 merge sorted lists with heap

Author: teckoo

_posts/coding/leetcode/2021-01-01-23-merge-k-sorted-lists.md

@@ -0,0 +1,66 @@
+---
+layout: leetcode
+title: "23. Merge k Sorted Lists"
+categories: [leetcode]
+---
+
+[Leetcode Link](https://leetcode.com/problems/merge-k-sorted-lists/)
+
+
+You are given an array of k linked-lists lists, each linked-list is sorted in ascending order.
+
+Merge all the linked-lists into one sorted linked-list and return it.
+
+```
+Example 1:
+
+Input: lists = [[1,4,5],[1,3,4],[2,6]]
+Output: [1,1,2,3,4,4,5,6]
+Explanation: The linked-lists are:
+[
+  1->4->5,
+  1->3->4,
+  2->6
+]
+merging them into one sorted list:
+1->1->2->3->4->4->5->6
+Example 2:
+
+Input: lists = []
+Output: []
+Example 3:
+
+Input: lists = [[]]
+Output: []
+ 
+
+Constraints:
+
+k == lists.length
+0 <= k <= 10^4
+0 <= lists[i].length <= 500
+-10^4 <= lists[i][j] <= 10^4
+lists[i] is sorted in ascending order.
+The sum of lists[i].length won't exceed 10^4.
+```
+
+# Solution
+
+The idea is useful when sorting or finding the median value of multiple disks which cannot be loaded into memory.
+
+```python
+import heapq
+
+def mergeKLists(self, lists: List[ListNode]) -> ListNode:
+    h = [(head.val, idx, head) for idx, head in enumerate(lists) if head]
+    heapq.heapify(h)
+    dummy = ListNode()
+    last = dummy
+    while h:
+        val, idx, node = heapq.heappop(h)
+        last.next = node
+        last = last.next
+        if node.next:
+            heapq.heappush(h, (node.next.val, idx, node.next))
+    return dummy.next
+```

media/js/tag-search-data.js

@@ -87,14 +87,22 @@ var problems = {
         templates: ["merge_sort"],
         notes: "Use dummy head",
     },
-     "22": {
+    "22": {
         name: "22. Generate Parentheses",
         url: "/leetcode/22-generate-parentheses",
         tags: ["backtrack", "array"],
         keywords: ["parentheses", "lv_3"],
         templates: ["backtrack"],
         notes: "Backtrack template. convert to validate the combinations of (). ",
     },
+    "23": {
+        name: "23. Merge k Sorted Lists",
+        url: "/leetcode/23-merge-k-sorted-lists",
+        tags: ["linked_list", "heap"],
+        keywords: ["merge lists", "lv_1"],
+        templates: [],
+        notes: "Heap, add heads of lists into a heap, O(NlogK)",
+    },
     "25": {
         name: "25. Reverse Nodes in k-Group",
         url: "/leetcode/25-reverse-nodes-in-k-group",