Sum Root to Leaf Numbers

Problem:

Given a binary tree containing digits from 0-9 only, each root-to-leaf path could represent a number.

An example is the root-to-leaf path 1->2->3 which represents the number 123.

Find the total sum of all root-to-leaf numbers.

For example,

    1
   / \
  2   3

The root-to-leaf path 1->2 represents the number 12.
The root-to-leaf path 1->3 represents the number 13.

Return the sum = 12 + 13 = 25.

Solution:

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public int sumNumbers(TreeNode root) {
        if(root==null)
           return 0;
        int sum = 0;
        
        Stack<TreeNode> st = new Stack<>();
        st.push(root);
    
        while(!st.empty())
        {
             TreeNode node = st.pop();
             while(node.left!=null)
             {
                 node.left.val += node.val*10;
                 if(node.right!=null)
                 {
                     node.right.val += node.val*10;
                     st.push(node.right);
                 }
                 node = node.left;
             }
             if(node.right!=null)
             {
                 node.right.val += node.val*10;
                 st.push(node.right);
              }
              else 
             {
                 sum += node.val;
             }
         }
    
        return sum;
    }
}