Description
You are given the root of a binary tree where each node has a value in the range [0, 25] representing the letters 'a' to 'z'.
Return the lexicographically smallest string that starts at a leaf of this tree and ends at the root.
As a reminder, any shorter prefix of a string is lexicographically smaller.
- For example,
"ab"is lexicographically smaller than"aba".
A leaf of a node is a node that has no children.
Example 1:
Input: root = [0,1,2,3,4,3,4] Output: "dba"
Example 2:
Input: root = [25,1,3,1,3,0,2] Output: "adz"
Example 3:
Input: root = [2,2,1,null,1,0,null,0] Output: "abc"
Constraints:
- The number of nodes in the tree is in the range
[1, 8500]. 0 <= Node.val <= 25
Solution
Python3
# 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 smallestFromLeaf(self, root: Optional[TreeNode]) -> str:
res = None
def go(node, curr):
nonlocal res
if not node: return
curr += chr(ord('a') + node.val)
if not node.left and not node.right:
if res is None or curr[::-1] < res:
res = curr[::-1]
else:
go(node.left, curr)
go(node.right, curr)
go(root, "")
return res