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) };`