```
int sumOfLeftLeaves(TreeNode* root) {
if(!root) return 0;
return helper(root->left) + sumOfLeftLeaves(root->right);
}
int helper(TreeNode* root){
if(!root) return 0;
if(!root->left && !root->right) return root->val;
return helper(root->left) + sumOfLeftLeaves(root->right);
}
```