```
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
/*
Basic idea: the sum of a node = f(leftSubTree) + f(rightSubtree)
However we need to distinguish between left leaf or right leaf
*/
public class Solution {
public int sumOfLeftLeaves(TreeNode root) {
return helper(root, false);
}
private int helper(TreeNode root, boolean isLeft){
if(root == null) return 0;
/*This node is a leaf, if it's a left leaf, we return the value
if it's a right leaf we return 0 since right leaf doesn't count*/
if(root.left == null && root.right == null){
if(isLeft){
return root.val;
}
return 0;
}
return helper(root.left, true) + helper(root.right, false);
}
}````
```