404\. Sum of Left Leaves

Easy

Given the `root` of a binary tree, return the sum of all left leaves.

**Example 1:**

![](https://assets.leetcode.com/uploads/2021/04/08/leftsum-tree.jpg)

**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`