Path Sum - Where am I going wrong?

Question

Leetcode problem #112 - Path Sum

Given the root of a binary tree and an integer targetSum, return true if the tree has a root-to-leaf path such that adding up all the values along the path equals targetSum.

A leaf is a node with no children.

Here is my approach in English - I traverse the tree using DFS. When I reach the leaf node in my first else statement, I append the sum of path value to a list. Lastly, I check if my_list has the targetSum contained in it. If yes, then return True else return False.

# 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 hasPathSum(self, root: Optional[TreeNode], targetSum: int) -> bool:
        my_list = []
        def build_some(root, cur_sum):
            if not root or targetSum == 0:
                return False 

            elif root.left or root.right:
                cur_sum += root.val
                build_some(root.left, cur_sum)
                build_some(root.right, cur_sum)
            
            else:
                my_list.append(cur_sum+root.val)

            if targetSum in my_list:
                return True
            else:
                return False
        build_some(root, 0)

I am facing a problem with the following testcase:

              5
           /     \
         4        8
        /        /  \
      11        13    4
    /    \             \
   7      2             1

Here, targetSum is 22. The path [5,4,11,2] sums up to 22 and my code works fine in identifying this (which I verified using print(my_list)). Still, my program is returning False when it should return True. Where am I going wrong in this?

I tried using try and except in first else statement to stop the recursive calls if the solution is found but that did not work either.

Answer 1

The hasPathSum function is missing a return statement so it always returns None, which evaluates to False. The last line should be changed to be return build_some(root, 0).

(Python functions defined with def do not automatically return the value of the last expression the way that lambdas do.)