【Grind75Hard】10問目124. Binary Tree Maximum Path Sum

grind75_hard/10_124_Binary Tree Maximum Path Sum/level_1.py

@@ -0,0 +1,19 @@
+# DFSを使う
+# 子を2つ持てるのは根だけ
+class Solution:
+    def maxPathSum(self, root: Optional[TreeNode]) -> int:
+        max_sum = -float("inf")
+
+        def helper(node):
+            nonlocal max_sum
+            if not node:
+                return 0
+            left_sum = helper(node.left)
+            right_sum = helper(node.right)
+            one_child_val = max(node.val + left_sum, node.val + right_sum)
+            two_child_val = node.val + left_sum + right_sum
+            max_sum = max(max_sum, node.val, one_child_val, two_child_val)
+            return max(node.val, one_child_val)
+
+        helper(root)
+        return max_sum

grind75_hard/10_124_Binary Tree Maximum Path Sum/level_2.py

@@ -0,0 +1,20 @@
+# 子ノードが負の場合は、その子ノードを選ばない(0にする)ように修正
+# root.valを初期値にしておく
+# 一応rootがNoneの場合は0を返すようにする
+class Solution:
+    def maxPathSum(self, root: Optional[TreeNode]) -> int:
+        if not root:
+            return 0
+        max_sum = root.val
+
+        def helper(node):
+            nonlocal max_sum
+            if not node:
+                return 0
+            left_sum = max(0, helper(node.left))
+            right_sum = max(0, helper(node.right))
+            max_sum = max(max_sum, node.val + left_sum + right_sum)
+            return node.val + max(left_sum, right_sum)
+
+        helper(root)
+        return max_sum

grind75_hard/10_124_Binary Tree Maximum Path Sum/level_3.py

@@ -0,0 +1,16 @@
+
+class Solution:
+    def maxPathSum(self, root: Optional[TreeNode]) -> int:
+        max_sum = root.val
+
+        def helper(node):
+            nonlocal max_sum
+            if not node:
+                return 0
+            left_sum = max(0, helper(node.left))
+            right_sum = max(0, helper(node.right))
+            max_sum = max(max_sum, node.val + left_sum + right_sum)
+            return max(node.val + left_sum, node.val + right_sum)
+
+        helper(root)
+        return max_sum

grind75_hard/10_124_Binary Tree Maximum Path Sum/level_4.py

@@ -0,0 +1,15 @@
+# 最大値を引数で渡すように修正
+class Solution:
+    def maxPathSum(self, root: Optional[TreeNode]) -> int:
+
+        def helper(node, max_sum):
+            if not node:
+                return 0, max_sum
+            left_sum, max_sum = helper(node.left, max_sum)
+            right_sum, max_sum = helper(node.right, max_sum)
+            max_sum = max(max_sum, node.val + left_sum + right_sum)
+            max_one_child = node.val + max(left_sum, right_sum)
+            return max(0, max_one_child), max_sum
+
+        _, max_sum = helper(root, root.val)
+        return max_sum