解释：根据规则构造数字，而不是从头遍历一遍判断，这样可以避免很多不必要的计算。

运行效率：0秒，312KB。

代码如下，类似广度优先搜索：

#include<stdio.h>
#include<string.h>
#include <iostream>
#include<queue>
using namespace std;
//int toInt(char c){ return c-'0'; }
//char toChar(int i){ return i+'0'; }
struct Node{	
	Node* pre;
	int pos;
	char self;
	Node():pos(-1),pre(NULL)
	{		
	}
};
int getNcount(int n)
{	
	char strNum[10];
	int ret = 0;
	sprintf(strNum,"%d",n);
	int len = strlen(strNum);
	queue<Node*> Q;
	queue<Node*> Mem;
	for(char i = '1'; i <= strNum[0]; i++)
	{
		Node* n = new Node;
		n->self = i;
		n->pos = 0;
		Q.push(n);
		Mem.push(n);
	}
	while(!Q.empty())
	{
		Node* front = Q.front();
		Q.pop();
		if(front->pos == len-1)
		{
			ret++;
// 			char res[10];
// 			Node* tmp = front;
// 			int ide = 0;
// 			while(tmp != NULL)
// 			{
// 				res[ide++]=tmp->self;
// 				tmp = tmp->pre;
// 			}			
// 			for(int i = ide-1; i >=0; i--)
// 				cout << res[i];
// 			cout << ' ';
		}
		else
		{
			if(front->pre == NULL)
			{
				char bound = '9';	
				if(front->self == strNum[0])
				{
					bound = strNum[1];
				}
				for(char s = '0'; s <= bound; s++)
				{
					Node* n = new Node;
					n->self = s;
					n->pos = front->pos+1;
					n->pre = front;
					Q.push(n);
					Mem.push(n);
				}
			}
			else
			{
				int sign[3][2] = {{1,1},{1,-1},{-1,1}};
				bool duplicateTest[10]={false};
				for (int k = 0; k < 3; k++)
				{
					int selfInt = sign[k][0]*(front->pre->self-'0')+sign[k][1]*(front->self-'0');
					if(selfInt < 0 || selfInt > 9)
					{
						continue;
					}
					if(duplicateTest[selfInt] == true)
						continue;
					else
						duplicateTest[selfInt] = true;
					Node* n = new Node;
					n->pre = front;
					n->pos = front->pos+1;					
					n->self = selfInt+'0';
					Node* tmp = n->pre;
					bool canPush = false;
					if(n->self <= strNum[n->pos])
						canPush = true;
					else
					{
						while(tmp != NULL)
						{
							if(tmp->self < strNum[tmp->pos])
							{
								canPush = true;
								break;
							}
							tmp = tmp->pre;
						}
					}					
					if(canPush)
					{
						Q.push(n);
						Mem.push(n);
					}
					else
					{
						delete n;
					}
				}
			}
		}
	}
	while (!Mem.empty())
	{
		Node* front = Mem.front();
		Mem.pop();
		delete front;
	}
	return ret;
}
int main()
{
	int N;
	while(cin >> N)
	{
		if(N < 100)
		{
			cout << N << endl;
			continue;
		}
		int count = 0;
		count = getNcount(N);
		char str[10];
		sprintf(str,"%d",N);
		int len = strlen(str);
		len--;
		while(len > 2)
		{
			char tmp[10];
			int i;
			for(i = 0; i < len; i++)
				tmp[i] = '9';
			tmp[i]='\0';
			int n;
			sscanf(tmp,"%d",&n);
			count += getNcount(n);
			len--;
		}
		if(len == 2)
			count += 99;
		cout << count << endl;
	}
}

sicily 2010 H number