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.

这个题目很简单，可用递归做：

class Solution {
public:
    int sumNumbers(TreeNode *root) {
        sum = 0;
        sumNumbers(root, 0);
        return sum;
    }

private:
    int sum;

    void sumNumbers(TreeNode *root, int num) {
        if (!root) return;

        num = num*10 + root->val;
        if (root->left == nullptr && root->right == nullptr) sum += num;
        if (root->left) sumNumbers(root->left, num);
        if (root->right) sumNumbers(root->right, num);
    }
};

LeetCode[Tree]: Sum Root to Leaf Numbers