1
var sumOfLeftLeaves = function (root) {
2
  let total = 0;
3

4
  const go = (node, isLeft) => {
5
    if (isLeft && !node.left && !node.right) {
6
      total += node.val;
7
      return;
8
    }
9
    if (node.left) go(node.left, true);
10
    if (node.right) go(node.right, false);
11
  };
12

13
  go(root, false);
14

15
  return total;
16
};