129. Sum Root to Leaf Numbers

题目描述

Given a binary tree containing digits from0-9only, each root-to-leaf path could represent a number.

An example is the root-to-leaf path1->2->3which represents the number123.

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

For example, 1

   / \
  2   3

The root-to-leaf path1->2represents the number12.
The root-to-leaf path1->3represents the number13.

Return the sum = 12 + 13 =25.

解题方法

透過sum * 10 + root.val

    public int sumNumbers(TreeNode root){
        return sum(root, 0);
    }

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

        int left = sum(root.left, sum * 10 + root.val);
        int right = sum(root.right, sum * 10 + root.val);
        return left + right;
    }