Have a variable to indicate the side. At every node, check the following,

if you are a left leaf, (with both children as null and side as 1), then return ur value

else return the sum of values returned by recursive left and right call

```
public int sumOfLeftLeaves(TreeNode root) {
return helper(root,-1);
}
public int helper(TreeNode root, int side){
if(root == null)return 0;
if( (root.left == null) && (root.right == null) && side == 1 ){
return root.val;
}
else{
return (helper(root.left,1)+ helper(root.right,0));
}
}
}
```