leetcode979

 1 class Solution(object):
 2     def __init__(self):
 3         self.res = 0
 4 
 5     def dfs(self,root):
 6         if not root: 
 7             return 0
 8         left = self.dfs(root.left)
 9         right = self.dfs(root.right)
10         self.res += abs(left) + abs(right)
11         return root.val + left + right - 1
12 
13     def distributeCoins(self, root: TreeNode) -> int:
14         self.dfs(root)
15         return self.res