DaleStudy#286 Merge k Sorted Lists

Author: forest000014

merge-k-sorted-lists/forest000014.java

@@ -0,0 +1,50 @@
+/*
+# Time Complexity: O(nlogk)
+  - n은 lists[i].length의 합
+# Space Complexity: O(k)
+  - pq에는 최대 k개의 원소가 저장됨
+*/
+/**
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     int val;
+ *     ListNode next;
+ *     ListNode() {}
+ *     ListNode(int val) { this.val = val; }
+ *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
+ * }
+ */
+class Solution {
+
+    public ListNode mergeKLists(ListNode[] lists) {
+        if (lists.length == 0) return null;
+
+        PriorityQueue<ListNode> pq = new PriorityQueue<>(new Comparator<ListNode>() {
+            @Override
+            public int compare(ListNode n1, ListNode n2) {
+                return n1.val - n2.val;
+            }
+        });
+        for (int i = 0; i < lists.length; i++) {
+            if (lists[i] == null) continue;
+            pq.offer(lists[i]);
+        }
+
+        ListNode head = next(pq);
+        ListNode curr = head;
+
+        while (!pq.isEmpty()) {
+            curr.next = next(pq);
+            curr = curr.next;
+        }
+
+        return head;
+    }
+
+    private ListNode next(PriorityQueue<ListNode> pq) {
+        ListNode top = pq.poll();
+        if (top == null) return null;
+        if (top.next != null) pq.offer(top.next);
+        return top;
+    }
+}