LeetCode: Sum Root to Leaf Numbers 解题报告

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.

SOLUTION 1:

使用递归完成。我们要做的是把左右子树的path加起来。

base case: root是叶子节点时，直接计算path。我们用pre保留上一层计算出来的数字的值。那么到了当前根节点应该就是

把上一层的值*10再加上root的值。

 1 /**
 2  * Definition for binary tree
 3  * public class TreeNode {
 4  *     int val;
 5  *     TreeNode left;
 6  *     TreeNode right;
 7  *     TreeNode(int x) { val = x; }
 8  * }
 9  */
10 public class Solution {
11     public int sumNumbers(TreeNode root) {
12         return dfs(root, 0);
13     }
14     
15     public int dfs(TreeNode root, int pre) {
16         if (root == null) {
17             return 0;
18         }
19         
20         int cur = pre * 10 + root.val;
21         if (root.left == null && root.right == null) {
22             return cur;
23         }
24         
25         return dfs(root.left, cur) + dfs(root.right, cur);
26     }
27 }

View Code

GITHUB:

https://github.com/yuzhangcmu/LeetCode_algorithm/blob/master/tree/SumNumbers_1208_2014.java