学步园

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.

题目解析：

还是递归求解，当递归的时候，传入这一层计算的结果值，供下层计算。当叶节点的时候，才能就保存退出。不能左子树或右子树为空，保存退出，这样会重复计算！考虑时候应该想全面一些。

class Solution {
public:
    int sumNumbers(TreeNode *root) {
        if(root == NULL)
            return 0;

        int res = 0;
        countSum(root,0);
        for(int i = 0;i < arr.size();i++)
            res += arr[i];
        return res;
    }
    void countSum(TreeNode *root,int sum){
        sum = sum*10 + root->val;
        if(root->left == NULL && root->right == NULL){
            arr.push_back(sum);
            return ;
        }
        if(root->left)
            countSum(root->left,sum);
        if(root->right)
            countSum(root->right,sum);
    }
private:
    vector<int> arr;
};