[Leet Code] Find Bottom Left Tree Value

Leetcode: https://leetcode.com/problems/find-bottom-left-tree-value/

Problem:

Given the root of a binary tree, return the leftmost value in the last row of the tree.

Example 1:

Input: root = [2,1,3]
Output: 1

Example 2:

Input: root = [1,2,3,4,null,5,6,null,null,7]
Output: 7

Solution:

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution(object):
    def findBottomLeftValue(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        if root:
            elements = [[root.val]]
            stack = [root]
            self.helper(stack, elements)
            length = len(elements) - 1
            for i in range(len(elements)):
                if i == length:
                    return elements[i][0]
            
    def helper(self, stack, elements):
        while len(stack) > 0:
            values = []
            length = len(stack)
            for i in range(length):
                node = stack.pop(0)
                if node.left:
                    stack.append(node.left)
                    values.append(node.left.val)
                if node.right:
                    stack.append(node.right)
                    values.append(node.right.val)
            if values != []:
                elements.append(values)