LeetCode in Python 366. Find Leaves of Binary Tree

# 366. Find Leaves of Binary Tree
# Example:
# Given binary tree
#           1
#          / \
#         2   3
#        / \
#       4   5
# Returns [4, 5, 3], [2], [1].
class Solution(object):
    ans = []
    def findLeaves(self, root):
        dfs(root)
        return self.ans

    def dfs(root):
        if not root:
            return 0
        left = dfs(root.left)
        right = dfs(root.right)
        depth = max(left, right)
        if depth == len(ans):
            self.ans.append([root.val])
        else:
            self.ans[-1].append(root.val)
        return depth+1

每次调用dfs函数，得到返回值的时候就是加入新解的时候，根据depth来判断这个解属于新的一层还是已有层。
此题的逻辑非常符合深度优先搜索的本质：到达叶子节点后逐级回溯