104_max_depth_of_binary_tree.py

'''
104. Maximum Depth of Binary Tree
Given a binary tree, find its maximum depth.

The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

Note: A leaf is a node with no children.

Example:

Given binary tree [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7
return its depth = 3.

# 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(object):
    def maxDepth(self, root):
        # recursion
        if root is None:
            return 0
        else:
            left_height = self.maxDepth(root.left)
            right_height = self.maxDepth(root.right)
            return max(left_height, right_height) + 1

    # solution 2
        # stack using BFS
        stack = []
        if root is not None:
            stack.append((1, root))

        depth = 0
        while stack != []:
            current_depth, root = stack.pop()
            if root is not None:
                depth = max(depth, current_depth)
                stack.append((current_depth + 1, root.left))
                stack.append((current_depth + 1, root.right))

        return depth


if __name__ == '__main__':
    # begin
    s = Solution()
    print(s.maxDepth([3, 9, 20, null, null, 15, 7]))