動機

從bottom重建tree

以後是不是應該整理dfs在tree上的用法?

Problem

Given the root of a binary tree and two integers val and depth, add a row of nodes with value val at the given depth depth.

Note that the root node is at depth 1.

The adding rule is:

  • Given the integer depth, for each not Leetcode tree node cur at the depth depth - 1, create two tree nodes with value val as cur's left subtree root and right subtree root.
  • cur's original left subtree should be the left subtree of the new left subtree root.
  • cur's original right subtree should be the right subtree of the new right subtree root.
  • If depth == 1 that means there is no depth depth - 1 at all, then create a tree node with value val as the new root of the whole original tree, and the original tree is the new root's left subtree.

 

Example 1:

Input: root = [4,2,6,3,1,5], val = 1, depth = 2Output: [4,1,1,2,null,null,6,3,1,5]

Example 2:

Input: root = [4,2,null,3,1], val = 1, depth = 3Output: [4,2,null,1,1,3,null,null,1]

 

Constraints:

  • The number of nodes in the tree is in the range [1, 104].
  • The depth of the tree is in the range [1, 104].
  • -100 <= Node.val <= 100
  • -105 <= val <= 105
  • 1 <= depth <= the depth of tree + 1

sol

class Solution:
    def addOneRow(self, root: Optional[TreeNode], val: int, depth: int, left = True) -> Optional[TreeNode]:
        if depth == 0:
            return root
        elif depth == 1:
            ret = TreeNode(val)
            (ret.left if left else ret.right) = root
            return ret
        else:
            if not root:
                return root
            else:
                root.left = self.addOneRow(root.left, val, depth-1, True)
                root.right = self.addOneRow(root.right, val, depth-1, False)
                return root