【Grind75Hard】8問目 23. Merge k Sorted Lists

grind75_hard/08_23_Merge k Sorted Lists/level_1.py

@@ -0,0 +1,40 @@
+# 要素を全て入れて、sortしてからlinked listに変換
+class Solution:
+    def mergeKLists(self, lists: List[Optional[ListNode]]) -> Optional[ListNode]:
+        lists_val = []
+        for node in lists:
+            while node:
+                lists_val.append(node.val)
+                node = node.next
+        lists_val.sort()
+        sentinel = ListNode()
+        node = sentinel
+        for i in lists_val:
+            node.next = ListNode(i)
+            node = node.next
+
+        return sentinel.next
+
+
+# 各linked listの先頭を比較していく
+class Solution:
+    def mergeKLists(self, lists: List[Optional[ListNode]]) -> Optional[ListNode]:
+        sentinel = ListNode()
+        node = sentinel
+        while lists:
+            min_val = float("inf")
+            for i in range(len(lists)):
+                if not lists[i] or lists[i].val > min_val:
+                    continue
+                min_val = lists[i].val
+                min_index = i
+            if min_val == float("inf"):
+                break
+            node.next = ListNode(min_val)
+            node = node.next
+            min_node = lists.pop(min_index)
+            min_node = min_node.next
+            if min_node:
+                lists.append(min_node)
+
+        return sentinel.next

grind75_hard/08_23_Merge k Sorted Lists/level_2.py

@@ -0,0 +1,19 @@
+# heapqに先頭の値だけを入れていく
+class Solution:
+    def mergeKLists(self, lists: List[Optional[ListNode]]) -> Optional[ListNode]:
+        min_heap = []
+        for i, node in enumerate(lists):
+            if not node:
+                continue
+            heapq.heappush(min_heap, (node.val, i, node))
+        sentinel = ListNode()
+        node = sentinel
+        while min_heap:
+            val, index, smallest_head = heapq.heappop(min_heap)
+            node.next = ListNode(val)
+            node = node.next
+            if smallest_head.next:
+                heapq.heappush(
+                    min_heap, (smallest_head.next.val, index, smallest_head.next)
+                )
+        return sentinel.next

grind75_hard/08_23_Merge k Sorted Lists/level_3.py

@@ -0,0 +1,18 @@
+class Solution:
+    def mergeKLists(self, lists: List[Optional[ListNode]]) -> Optional[ListNode]:
+        min_heap = []
+        for i, node in enumerate(lists):
+            if not node:
+                continue
+            heapq.heappush(min_heap, (node.val, i, node))
+        sentinel = ListNode()
+        node = sentinel
+        while min_heap:
+            val, index, smallest_head = heapq.heappop(min_heap)
+            node.next = ListNode(val)
+            node = node.next
+            if smallest_head.next:
+                heapq.heappush(
+                    min_heap, (smallest_head.next.val, index, smallest_head.next)
+                )
+        return sentinel.next

grind75_hard/08_23_Merge k Sorted Lists/level_4.py

@@ -0,0 +1,52 @@
+class Solution:
+    def mergeKLists(self, lists: List[Optional[ListNode]]) -> Optional[ListNode]:
+        min_heap = []
+        for i, node in enumerate(lists):
+            if not node:
+                continue
+            # node.valが同値の場合にListNodeで比較しないようにindexをheapに追加
+            heapq.heappush(min_heap, (node.val, i, node))
+        sentinel = ListNode()
+        node = sentinel
+        while min_heap:
+            val, index, smallest_head = heapq.heappop(min_heap)
+            node.next = smallest_head
+            node = node.next
+            if smallest_head.next:
+                heapq.heappush(
+                    min_heap, (smallest_head.next.val, index, smallest_head.next)
+                )
+        return sentinel.next
+
+
+# マージソートをイメージした解法
+# 先頭の2つのリストをマージしていき、最後の1つになるまで繰り返す
+class Solution:
+    def mergeKLists(self, lists: List[Optional[ListNode]]) -> Optional[ListNode]:
+        def merge_two_lists(list_1, list_2):
+            sentinel = ListNode()
+            node = sentinel
+            while list_1 and list_2:
+                if list_1.val < list_2.val:
+                    node.next = list_1
+                    list_1 = list_1.next
+                else:
+                    node.next = list_2
+                    list_2 = list_2.next
+                node = node.next
+            if not list_1:
+                node.next = list_2
+            else:
+                node.next = list_1
+            return sentinel.next
+
+        list_queue = deque(lists)
+        if not list_queue:
+            return None
+        while 1:
+            list_1 = list_queue.popleft()
+            if not list_queue:
+                return list_1
+            list_2 = list_queue.popleft()
+            mearged_list = merge_two_lists(list_1, list_2)
+            list_queue.append(mearged_list)