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：
直接dfs，将到达叶节点代表的数字加起来即可。
void dfs(long long input, TreeNode* node, long long &result)
{
assert(node != NULL);
long long cur = input * 10 + node->val;
if(node->left == NULL && node->right == NULL) //leaf
{
result += cur;
return;
}
if(node->left)
dfs(cur, node->left, result);
if(node->right)
dfs(cur, node->right, result);
}
int sumNumbers(TreeNode *root)
{
if(root == NULL) return 0;
long long result = 0L;
dfs(0, root, result);
return (int)result;
}