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.
Note:A leaf is a node with no children.
给定一棵二叉树，每个根节点到叶子结点的遍历可以表示一个数字，如1->2->3可表示123。求所有遍历加起来的和。
递归。从上到下遍历，在已知正在遍历的节点root和遍历过的节点的情况下，写一个函数去计算接下来遍历的值。
eg.当遍历到root：10*x+root->val
class Solution {
public:
int sumNumbers(TreeNode* root) {
if(!root) return 0;
return sumNum(root, 0);
}
int sumNum(TreeNode* root, int x){
if(!root->left && !root->right){
return 10*x + root->val;
}
int val = 0;
if(root->left){
val += sumNum(root->left, 10*x+root->val);
}
if(root->right){
val += sumNum(root->right, 10*x+root->val);
}
return val;
}
};