129. Sum Root to Leaf Numbers 解题记录

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. 

解题思路：

这题我采用的是递归的方法，按照题目的描述，每遇到一次分岔路就会衍生出两个数相加，于是我就利用递归，从二叉树的最底分支开始相加，一层层往上叠加，最终得到所求的数。

代码：

/**
 * 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 pathSum(TreeNode* root, int sum){
        if(!root)  
        //空节点返回0
            return 0;
        if(!root->left && !root->right)
        //到最底节点
            return sum*10+root->val;
        return pathSum(root->left, sum*10+root->val)+pathSum(root->right, sum*10+root->val);  //存在至少一个节点
    }
    int sumNumbers(TreeNode* root) {
        return pathSum(root, 0);
    }
};