We should check if this is the left leave two levels above.

```
class Solution {
public:
int sumOfLeftLeaves(TreeNode* root) {
if ( root == nullptr ) return 0;
int ans = 0;
if ( root->left != nullptr && root->left->left == nullptr && root->left->right == nullptr) ans = root->left->val;
return ans + sumOfLeftLeaves(root->left) + sumOfLeftLeaves(root->right);
}
};
```