513. Find Bottom Left Tree Value

solution.py

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def _left_first_traverse(self, node, depth):
        if node is None:
            return

        if depth > self.max_depth:
            self.max_depth = depth
            self.result_node = node       
        
        # Recurse left side first to ensure the
        # leftmost node is found first at a new depth
        self._left_first_traverse(node.left, depth + 1)
        self._left_first_traverse(node.right, depth + 1)

    def findBottomLeftValue(self, root: Optional[TreeNode]) -> int:
        self.max_depth = 0
        self.result_node = None

        self._left_first_traverse(root, 1)
        return self.result_node.val