leetcode——988. 从叶结点开始的最小字符串

class Solution:
    def smallestFromLeaf(self, root: TreeNode) -> str:
        self.ans="~"
        def dfs(node,A):
            if node:
                A.append(chr(node.val+ord(‘a‘)))
                if not node.left and not node.right:
                    self.ans=min(self.ans,‘‘.join(reversed(A)))
                dfs(node.left,A)
                dfs(node.right,A)
                A.pop()
        dfs(root,[])
        return self.ans