Sum of Left Leaves — LeetCode 404 Python Solution

The problem

Given the root of a binary tree, return the sum of all left leaves. A leaf is a node with no children.

Example

Input

root = [3,9,20,null,null,15,7]

Output

24

Explanation

There are two left leaves in the binary tree, with values 9 and 15 respectively.

Python solution

# 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 sumOfLeftLeaves(self, root: Optional[TreeNode]) -> int:
        if root is None:
            return 0
        ans = self.sumOfLeftLeaves(root.right)
        if root.left:
            if root.left.left == root.left.right:
                ans += root.left.val
            else:
                ans += self.sumOfLeftLeaves(root.left)
        return ans

Complexity

Pattern: Tree Traversal

Choose the order — preorder, inorder, postorder, level — and the problem solves itself. LeetCode 404. Sum of Left Leaves is filed here because LeetCode tags it Tree and Binary Tree, which is the vocabulary this hub collects.

The tree traversal guide has the Python template for the pattern and the 225 LeetCode problems that use it.

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