Sum Root to Leaf Numbers 解答

Question

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

基本思路是Recursion。我们可以这样看待问题：对于非叶子结点，我们更改它的数值（prev * 10 + currentNode.val）。然后将所有叶子结点的值相加即得到结果。

 1 /**
 2  * Definition for a binary tree node.
 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, 0);
13     }
14     
15     private int dfs(TreeNode root, int prev, int sum) {
16         if (root == null) {
17             return sum;
18         }
19         prev = prev * 10 + root.val;
20         if (root.left == null && root.right == null) {
21             sum += prev;
22             return sum;
23         }
24         return dfs(root.left, prev, sum) + dfs(root.right, prev, sum);
25     }
26 }