Given the root of a binary tree, return the sum of all **left leaves**. A left leaf is a leaf node that is the left child of its parent.
**Note:** The tree is given as a level-order array where the node at index `i` has its left child at index `2*i + 1` and its right child at index `2*i + 2`, with `null` for missing nodes (trailing positions may be omitted).
For example, `[3, 9, 20, null, null, 15, 7]` represents:
```
3
/ \
9 20
/ \
15 7
```
The left leaves are 9 and 15, so the answer is 24.
Example 1
Input:root = [3, 9, 20, null, null, 15, 7]
Output:24
Explanation:There are two left leaves, 9 and 15.
Example 2
Input:root = [1]
Output:0
Explanation:The root is not a left leaf.
Example 3
Input:root = []
Output:0
Constraints
- The number of nodes in the tree is in the range [0, 1000].
- -1000 <= Node.val <= 1000