Python recursion didn't give correct result for "sum of left leaves"

I wrote a code for the leetcode problem "108. Sum of Left Leaves" using recursion. It didn't give me the expected result.

# Definition for a binary tree node.
# class TreeNode(object):
# def __init__(self, x):
# self.val = x
# self.left = None
# self.right = None
class Solution(object): def sumOfLeftLeaves(self, root): """ :type root: TreeNode :rtype: int """ if root is None: return 0 ans = 0 if root.left and not root.left.left and not root.left.right: ans += root.left.val self.sumOfLeftLeaves(root.left) self.sumOfLeftLeaves(root.right) return ans

When the input is [3,9,20,null,null,15,7]

I expected a 24 to be returned, but the code only gave me 9

1 Answer

You aren't adding the results of the leaves below the root. You need this:

class Solution(object): def sumOfLeftLeaves(self, root): """ :type root: TreeNode :rtype: int """ if root is None: return 0 ans = 0 if root.left and not root.left.left and not root.left.right: ans += root.left.val ans += self.sumOfLeftLeaves(root.left) + self.sumOfLeftLeaves(root.right) return ans