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].
