leetcode Sum Root to Leaf Numbers

题目

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.

思路

这个题目典型的递归题目，关于递归注意与回溯区别开来。在递归的时候用一个变量s保存到当前节点时，前面节点的和 , 当一个节点是叶子节点的时候，将当前和累加入到总和中去，递归返回的条件是节点为空。

代码

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    private int total = 0;
    public int sumNumbers(TreeNode root) {
        helper(root,0);
        return total;
    }
    
    public void helper(TreeNode root,int s){
        if(root == null) return;
        s = s*10 + root.val;
        if(root.left == null && root.right == null){
            total += s;
            return;
        }
        helper(root.left,s);
        helper(root.right,s);
    }
}

PS：最近在刷题，希望看到自己刷到300+的时候是个什么水平吧。

吐槽：优快云编辑更新了，字体更改大小，部分文字格式没起作用。