#leetcode#Sum Root to Leaf Numbers

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.

分析： 从root到leaf一条路径上的和之前是做过的，这里稍微变化一下， 上一层的value * 10 再加当前层 value 就是所要维护的值， 利用Java pass by value的特性， 用一个List<Integer> res来保存两个变量， res.get(0)回溯跟踪得到某一条root到leaf的数字是多少， root.get(1)维护遍历过的各个路径得到的数字和。

/**
 * Definition for a binary tree node.
 * 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;
        }
        
        List<Integer> res = new ArrayList<>();
        res.add(0);
        res.add(0);
        helper(res, root);
        return res.get(1);
    }
    
    private void helper(List<Integer> res, TreeNode root){
        if(root == null){
            return;
        }
        
        if(root.left == null && root.right == null){
            res.set(1, res.get(1) + res.get(0) * 10 + root.val);
            return;
        }
        
        res.set(0, res.get(0) * 10 + root.val);
        helper(res, root.left);
        helper(res, root.right);
        res.set(0, (res.get(0) - root.val) / 10);
    }
}