[leetcode] 129. 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.

解法一：

recursion的解法，对应dfs。对每一个node来说，如果它是left, 应该返回sum*10+val，sum是之前拿到的值。不然，继续迭代。

/** * Definition for a binary tree node. * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    int sumNumbers(TreeNode* root) {        return sumNumbersDFS(root, 0);    }        int sumNumbersDFS(TreeNode* root, int sum){        if(!root) return 0;        sum = sum*10 + root->val;        if(!root->left && !root->right) return sum;        return sumNumbersDFS(root->left, sum) + sumNumbersDFS(root->right,sum);    }};