Simple DFS solution

```
public class Solution {
public int sumOfLeftLeaves(TreeNode root) {
return helper(root, false);
}
public int helper(TreeNode root, boolean isLeft) {
if (root == null) return 0;
if (root.left == null && root.right == null && isLeft) return root.val;
int left = helper(root.left, true);
int right = helper(root.right, false);
return left + right;
}
}
```