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