UVa 10427 - Naughty Sleepy Boys

　　题目大意：从1开始往后写数字，构成一个如下的字符串 123456789101112... 。求第n位的数字是多少。

　　找规律，按数字的位数可以构建一个类似杨辉三角的东西，求出第n位是哪个数的第几位即可。

 #include <cstdio>
 #include <cmath>

 int main()
 {
 #ifdef LOCAL
     freopen("in", "r", stdin);
 #endif
     int table[] = {};
     int base = ;
     for (int i = ; i <= ; i++)
     {
         table[i] = i * base;
         base *= ;
     }
     for (int i = ; i <= ; i++)
         table[i] += table[i-];
     int n;
     while (scanf("%d", &n) != EOF)
     {
         int k = ;
         while (table[k] < n)   k++;
         double t1 = 1.0 * (n - table[k-]) / k;
         int offset = (int)ceil(t1);  // n is offset-th num of k-th row
         int t2 = (n - table[k-] - ) % k + ;  // the t2-th digit of the num
         int t3 = ;
         for (int i  = ; i <= k-; i++)
             t3 = t3* + ;
         int m = t3 + offset;  // the compete num
         for (int i = ; i < k-t2; i++)   m /= ;
         printf("%d\n", m % );
     }
     return ;
 }