Binary Tree Leaf Sum.java

/*
Given a binary tree, calculate the sum of leaves.
    1
   / \
  2   3
 /
4
return 7.

*/
/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */
public class Solution {
    /**
     * @param root the root of the binary tree
     * @return an integer
     */
    private int sum=0;
    public int leafSum(TreeNode root) {
        // Write your code 
        helper(root);
        return sum;
        
    }
    public void helper(TreeNode root){
        if(root==null){
            return;
        }
        if(root.left==null && root.right==null){
            sum+=root.val;
        }
        helper(root.left);
        helper(root.right);
    }
}