Sum of Left Leaves

Find the sum of all left leaves in a given binary tree.

Example:

    3
   / \
  9  20
    /  \
   15   7

There are two left leaves in the binary tree, with values 9 and 15 respectively. Return 24.

Solution:

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {    
    public int sumOfLeftLeaves(TreeNode root) {
        return helper(root, null);
    }
    
    private int helper(TreeNode root, TreeNode parent) {
        if (root == null) return 0;
        if (root.left == null && root.right == null && parent != null && parent.left == root) return root.val;
        int left = helper(root.left, root);        
        int right = helper(root.right, root);
        return left + right;
    }
}