Recursively look at each node and take value of the it's left node, if the left node has no left or right child

```
var sumOfLeftLeaves = function(root) {
var val = 0;
if(!root)return 0;
if (root.left && !root.left.left && !root.left.right) val = root.left.val;
return val + sumOfLeftLeaves(root.left) + sumOfLeftLeaves(root.right);
};
```