Sum of Left Leaves

Easy

Given the root of a binary tree, return the sum of all left leaves — leaf nodes that are the left child of their parent.

Example 1

Input:  root = [3,9,20,null,null,15,7]
Output: 24
Explanation: Left leaves are 9 and 15; 9 + 15 = 24.

Example 2

Input:  root = [1]
Output: 0

Constraints

• 1 <= number of nodes <= 1000
• -1000 <= Node.val <= 1000