https://leetcode.com/problems/smallest-string-starting-from-leaf

smallest-string-starting-from-leaf.py

class Solution:
    def smallestFromLeaf(self, root: TreeNode) -> str:
        smallest_so_far: 'List[Tuple[int]]' = [(27,)]  # sentinel larger than any lowercase letter
        def explore(node: TreeNode, stack: 'List[int]') -> None:
            stack.append(node.val)
            if not node.left and not node.right:  # leaf
                smallest_so_far[0] = min(smallest_so_far[0], tuple(reversed(stack)))
            else:
                for child in (node.left, node.right):
                    if child:
                        explore(child, stack)
            stack.pop()
        
        explore(root, [])
        return "".join(chr(val + ord('a')) for val in smallest_so_far[0])