public class Solution
extends Object
404 - Sum of Left Leaves\.
Easy
Given the `root` of a binary tree, return the sum of all left leaves.
**Example 1:**
**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.
**Example 2:**
**Input:** root = [1]
**Output:** 0
**Constraints:**
* The number of nodes in the tree is in the range `[1, 1000]`.
* `-1000 <= Node.val <= 1000`