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

A leaf is a node with no children. A left leaf is a leaf that is the left child of another node.

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`