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 }

 

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