public int sumNumbers(TreeNode root) { int sum = 0; if (root == null) { return sum; } return preorderSumNumbers(root, sum); } public int preorderSumNumbers(TreeNode root, int sum) { if (root == null) return 0; sum = sum * 10 + root.val; if (root.left == null && root.right == null) { return sum; } return preorderSumNumbers(root.left, sum) + preorderSumNumbers(root.right, sum); }