/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number}
 */
var sumNumbers = function(root) {
    // base case
    if (!root) return 0;
    
    // if leaf
    if (!root.left && !root.right) return root.val;
    
   const sumSubTree = (root, partialSum) => {
        // base case;
        if (!root) return 0;
        
        let currentSum = partialSum * 10 + root.val;
        
        // leaf
        if (!root.left && !root.right) {
            return currentSum; 
        }
        
        return sumSubTree(root.left, currentSum) + sumSubTree(root.right, currentSum)
    }
    
    return sumSubTree(root.left, root.val) + sumSubTree(root.right, root.val)
};