/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    private int leftsum = 0;
    public int sumOfLeftLeaves(TreeNode root) {
        if(root==null) return 0;

        helper(root,0);
        return leftsum;
    }

    public void helper(TreeNode node, int k){
        if(k==1 && node.left==null && node.right==null)
        leftsum+=node.val;

        if(node.left!=null){
            helper(node.left,1);
        }

        if(node.right!=null){
            helper(node.right,0);
        }
    }
}