程序员面试金典 - 面试题 17.06. 2出现的次数（找递推规律）

示例:
输入: 25
输出: 9
解释: (2, 12, 20, 21, 22, 23, 24, 25)(注意 22 应该算作两次)

提示：
n <= 10^9

class Solution {
public:
    int numberOf2sInRange(int n) {
        long i = 1, count = 0, high, cur, low;
    	while(n/i)//遍历每个位
    	{
    		high = n/(10*i);//高位
    		cur = (n/i)%10;//当前位
    		low = n-(n/i)*i;//低位
    		if(cur < 2)
    			count += high*i;
    		else if(cur == 2)
    			count += high*i+low+1;
    		else
    			count += (high+1)*i;
    		i *= 10;
    	}
    	return count;
    }
};