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617 merge two binary trees #23

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948df6d
step12
kitano-kazuki Mar 18, 2026
8b90d0a
md
kitano-kazuki Mar 18, 2026
fefe916
step3 1st
kitano-kazuki Mar 18, 2026
fbb050d
step3 2nd
kitano-kazuki Mar 18, 2026
3fae2a4
md
kitano-kazuki Mar 18, 2026
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527 changes: 527 additions & 0 deletions memo.md
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32 changes: 32 additions & 0 deletions step1-1_recursion.py
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# solved 8:33
from typing import Optional
from copy import deepcopy


# Definition for a binary tree node.
# class TreeNode:
# def __init__(self, val=0, left=None, right=None):
# self.val = val
# self.left = left
# self.right = right


class Solution:
def mergeTrees(self, root1: Optional[TreeNode], root2: Optional[TreeNode]) -> Optional[TreeNode]:

def mergeTrees_helper(node1: Optional[TreeNode], node2: Optional[TreeNode]) -> Optional[TreeNode]:
if node1 is None:
return deepcopy(node2)
if node2 is None:
return deepcopy(node1)

merged_node = TreeNode(val=node1.val + node2.val)
merged_tree_left = mergeTrees_helper(node1.left, node2.left)
merged_tree_right = mergeTrees_helper(node1.right, node2.right)
merged_node.left = merged_tree_left
merged_node.right = merged_tree_right
return merged_node

return mergeTrees_helper(root1, root2)


55 changes: 55 additions & 0 deletions step1-2_dfs.py
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from typing import Optional
from copy import deepcopy


# Definition for a binary tree node.
# class TreeNode:
# def __init__(self, val=0, left=None, right=None):
# self.val = val
# self.left = left
# self.right = right
class Solution:
def mergeTrees(self, root1: Optional[TreeNode], root2: Optional[TreeNode]) -> Optional[TreeNode]:
if root1 is None:
return deepcopy(root2)
if root2 is None:
return deepcopy(root1)

merged_dummy_head = TreeNode()
tree1_frontier = [root1]
tree2_frontier = [root2]
merge_instructions = [(merged_dummy_head, "left")]
while tree1_frontier and tree2_frontier:
node1 = tree1_frontier.pop()
node2 = tree2_frontier.pop()
target, creation_side = merge_instructions.pop()
if node1 is None and node2 is None:
continue
if creation_side == "left":
if node1 is None:
target.left = deepcopy(node2)
continue
if node2 is None:
target.left = deepcopy(node1)
continue
merged_node = TreeNode(node1.val + node2.val)
target.left = merged_node
elif creation_side == "right":
if node1 is None:
target.right = deepcopy(node2)
continue
if node2 is None:
target.right = deepcopy(node1)
continue
merged_node = TreeNode(node1.val + node2.val)
target.right = merged_node
else:
raise ValueError("creation side should be either right or left")
tree1_frontier.append(node1.left)
tree2_frontier.append(node2.left)
merge_instructions.append((merged_node, "left"))
tree1_frontier.append(node1.right)
tree2_frontier.append(node2.right)
merge_instructions.append((merged_node, "right"))

return merged_dummy_head.left
51 changes: 51 additions & 0 deletions step1-3_bfs.py
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from typing import Optional
from collections import deque
from copy import deepcopy


# Definition for a binary tree node.
# class TreeNode:
# def __init__(self, val=0, left=None, right=None):
# self.val = val
# self.left = left
# self.right = right
class Solution:
def mergeTrees(self, root1: Optional[TreeNode], root2: Optional[TreeNode]) -> Optional[TreeNode]:
if root1 is None:
return deepcopy(root2)
if root2 is None:
return deepcopy(root1)

dummy_merged_root = TreeNode()
frontier = deque([(root1, root2, dummy_merged_root, "left")])
while frontier:
next_frontier = deque()
while frontier:
node1, node2, target_node, creation_side = frontier.popleft()
if node1 is None and node2 is None:
continue
if node1 is None:
if creation_side == "left":
target_node.left = deepcopy(node2)
continue
if creation_side == "right":
target_node.right = deepcopy(node2)
continue
if node2 is None:
if creation_side == "left":
target_node.left = deepcopy(node1)
continue
if creation_side == "right":
target_node.right = deepcopy(node1)
continue
merged_node = TreeNode(node1.val + node2.val)
if creation_side == "left":
target_node.left = merged_node
elif creation_side == "right":
target_node.right = merged_node
next_frontier.append((node1.left, node2.left, merged_node, "left"))
next_frontier.append((node1.right, node2.right, merged_node, "right"))
frontier = next_frontier
return dummy_merged_root.left


28 changes: 28 additions & 0 deletions step2-1_recursion.py
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from typing import Optional
from copy import deepcopy


# Definition for a binary tree node.
# class TreeNode:
# def __init__(self, val=0, left=None, right=None):
# self.val = val
# self.left = left
# self.right = right


class Solution:
def mergeTrees(self, root1: Optional[TreeNode], root2: Optional[TreeNode]) -> Optional[TreeNode]:
if root1 is None and root2 is None:
return None
if root1 is None:
return deepcopy(root2)
if root2 is None:
return deepcopy(root1)

merged_node = TreeNode(val=root1.val + root2.val)
merged_node.left = self.mergeTrees(root1.left, root2.left)
merged_node.right = self.mergeTrees(root1.right, root2.right)
return merged_node



57 changes: 57 additions & 0 deletions step2-2_dfs.py
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from typing import Optional
from copy import deepcopy


# Definition for a binary tree node.
# class TreeNode:
# def __init__(self, val=0, left=None, right=None):
# self.val = val
# self.left = left
# self.right = right
class Solution:
def mergeTrees(self, root1: Optional[TreeNode], root2: Optional[TreeNode]) -> Optional[TreeNode]:
if root1 is None and root2 is None:
return None
if root1 is None:
return deepcopy(root2)
if root2 is None:
return deepcopy(root1)

def get_merged_val(node1, node2):
val = 0
if node1 is not None:
val += node1.val
if node2 is not None:
val += node2.val
return val

def get_left(node):
if node is None:
return None
return node.left

def get_right(node):
if node is None:
return None
return node.right


merged_root = TreeNode()
frontier = [(root1, root2, merged_root)]
while frontier:
node1, node2, merged_node = frontier.pop()
merged_node.val = get_merged_val(node1, node2)

node1_left = get_left(node1)
node2_left = get_left(node2)
if node1_left is not None or node2_left is not None:
merged_node.left = TreeNode()
frontier.append((node1_left, node2_left, merged_node.left))

node1_right = get_right(node1)
node2_right = get_right(node2)
if node1_right is not None or node2_right is not None:
merged_node.right = TreeNode()
frontier.append((node1_right, node2_right, merged_node.right))

return merged_root
57 changes: 57 additions & 0 deletions step2-3_bfs.py
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from typing import Optional
from copy import deepcopy


# Definition for a binary tree node.
# class TreeNode:
# def __init__(self, val=0, left=None, right=None):
# self.val = val
# self.left = left
# self.right = right
class Solution:
def mergeTrees(self, root1: Optional[TreeNode], root2: Optional[TreeNode]) -> Optional[TreeNode]:
if root1 is None and root2 is None:
return None
if root1 is None:
return deepcopy(root2)
if root2 is None:
return deepcopy(root1)

def get_merged_val(node1, node2):
val = 0
if node1 is not None:
val += node1.val
if node2 is not None:
val += node2.val
return val

def get_left(node):
if node is None:
return None
return node.left

def get_right(node):
if node is None:
return None
return node.right

merged_root = TreeNode()
frontier = [(root1, root2, merged_root)]
while frontier:
next_frontier = []
while frontier:
node1, node2, merged_node = frontier.pop()
merged_node.val = get_merged_val(node1, node2)

node1_left, node2_left = get_left(node1), get_left(node2)
if node1_left is not None or node2_left is not None:
merged_node.left = TreeNode()
next_frontier.append((node1_left, node2_left, merged_node.left))
node1_right, node2_right = get_right(node1), get_right(node2)
if node1_right is not None or node2_right is not None:
merged_node.right = TreeNode()
next_frontier.append((node1_right, node2_right, merged_node.right))
frontier = next_frontier
return merged_root


27 changes: 27 additions & 0 deletions step3-1_recursion.py
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# 1st: 1:30
# 2nd: 1:20
from typing import Optional
from copy import deepcopy


# Definition for a binary tree node.
# class TreeNode:
# def __init__(self, val=0, left=None, right=None):
# self.val = val
# self.left = left
# self.right = right


class Solution:
def mergeTrees(self, root1: Optional[TreeNode], root2: Optional[TreeNode]) -> Optional[TreeNode]:
if root1 is None and root2 is None:
return None
if root1 is None:
return deepcopy(root2)
if root2 is None:
return deepcopy(root1)

merged_root = TreeNode(root1.val + root2.val)
merged_root.left = self.mergeTrees(root1.left, root2.left)
merged_root.right = self.mergeTrees(root1.right, root2.right)
return merged_root
53 changes: 53 additions & 0 deletions step3-2_dfs.py
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Original file line number Diff line number Diff line change
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# 1st: 3:28
# 2nd: 3:20
from typing import Optional
from copy import deepcopy


# Definition for a binary tree node.
# class TreeNode:
# def __init__(self, val=0, left=None, right=None):
# self.val = val
# self.left = left
# self.right = right
class Solution:
def mergeTrees(self, root1: Optional[TreeNode], root2: Optional[TreeNode]) -> Optional[TreeNode]:
if root1 is None and root2 is None:
return None
if root1 is None:
return deepcopy(root2)
if root2 is None:
return deepcopy(root1)

def get_merged_val(node1, node2):
val = 0
if node1 is not None:
val += node1.val
if node2 is not None:
val += node2.val
return val

def get_left(node):
if node is None:
return None
return node.left

def get_right(node):
if node is None:
return None
return node.right

merged_root = TreeNode()
frontier = [(root1, root2, merged_root)]
while frontier:
node1, node2, merged_node = frontier.pop()
merged_node.val = get_merged_val(node1, node2)
node1_left, node2_left = get_left(node1), get_left(node2)
if node1_left is not None or node2_left is not None:
merged_node.left = TreeNode()
frontier.append((node1_left, node2_left, merged_node.left))
node1_right, node2_right = get_right(node1), get_right(node2)
if node1_right is not None or node2_right is not None:
merged_node.right = TreeNode()
frontier.append((node1_right, node2_right, merged_node.right))
return merged_root
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