示例 1:
输入:
    2
   / \
  1   3
输出:
1
示例 2:
输入:
        1
       / \
      2   3
     /   / \
    4   5   6
       /
      7
输出:
7

from collections import deque
class Solution:
    def findBottomLeftValue(self, root):
        li, queue= [], deque([[root, 1]])
        while queue:
            for _ in range(len(queue)):
                child, index = queue.popleft()
                if len(li) < index:
                    li.append(child.val)
                index += 1
                if child.left:
                    queue.append([child.left, index])
                if child.right:
                    queue.append([child.right, index])
        return li.pop()

class Solution:
    def findBottomLeftValue(self, root):
        d = {}
        def dfs(child, index):
            if not child:
                return
            if index not in d:
                d[index] = child.val
            dfs(child.left, index + 1)
            dfs(child.right, index + 1)
        dfs(root, 1)
        return list(d.values()).pop()