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.

思路是借鉴了大牛们的想法。

类似先序便利，先根结点，再左子树结点，然后右子树结点。

int sumNumbersCore(TreeNode* node,int sum)
    {
        if (node == NULL)return 0;
        sum = sum * 10 + node->val;
        if (node->left == NULL && node->right == NULL) return sum;//叶结点

        return sumNumbersCore(node->left, sum) + sumNumbersCore(node->right, sum);         
    }
    int sumNumbers(TreeNode* root)
    {         
        return sumNumbersCore(root,0);
    }

[leetcode-129-Sum Root to Leaf Numbers]