Distribute Coins in Binary Tree — LeetCode 979 Python Solution

The problem

You are given the root of a binary tree with n nodes where each node in the tree has node.val coins. There are n coins in total throughout the whole tree.

Example

Input

root = [3,0,0]

Output

2

Explanation

From the root of the tree, we move one coin to its left child, and one coin to its right child.

Python solution

# 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 distributeCoins(self, root: Optional[TreeNode]) -> int:
        def dfs(root):
            if root is None:
                return 0
            left, right = dfs(root.left), dfs(root.right)
            nonlocal ans
            ans += abs(left) + abs(right)
            return left + right + root.val - 1

        ans = 0
        dfs(root)
        return ans

Complexity

Pattern: Tree Traversal

Choose the order — preorder, inorder, postorder, level — and the problem solves itself. LeetCode 979. Distribute Coins in Binary Tree is filed here because LeetCode tags it Tree and Binary Tree, which is the vocabulary this hub collects.

The tree traversal guide has the Python template for the pattern and the 225 LeetCode problems that use it.

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