Leetcode: Sum Root to Leaf Numbers

http://oj.leetcode.com/problems/sum-root-to-leaf-numbers/

// Tried many times to get accepted
// Take care:
// 1. root is NULL
// 2. node with one child
// 3. leaf
class Solution {
public:
    int sumNumbers(TreeNode *root, int last){
        if(root->left==NULL&&root->right==NULL) return last*10+root->val;
        else if(root->left==NULL) return sumNumbers(root->right,last*10+root->val);
        else if(root->right==NULL) return sumNumbers(root->left,last*10+root->val);
        else return sumNumbers(root->left,last*10+root->val)+sumNumbers(root->right,last*10+root->val);
    }
    int sumNumbers(TreeNode *root) {
        // Take care of the case that root is NULL
        if(root==NULL) return 0;
        else return sumNumbers(root,0);
    }
};