Find the sum of all left leaves in a given binary tree. There are two left leaves in the binary tree, with values 9 and 15 respectively. Return 24.

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number}
 */
var sumOfLeftLeaves = function(root) {
    if(root === null) return null
    if(root.left && !root.left.left && !root.left.right){
        return root.left.val + sumOfLeftLeaves(root.right)
    }
    return sumOfLeftLeaves(root.left) + sumOfLeftLeaves(root.right)
};