cf B. Making Sequences is Fun

http://codeforces.com/contest/373/problem/B

用二分枚举长度就可以。

 1 #include <cstdio>

 2 #include <cstring>

 3 #include <algorithm>

 4 #define LL __int64

 5 using namespace std;

 6 

 7 LL w,m,k;

 8 

 9 bool ok(LL c)

10 {

11     LL ans=0;

12     LL x2=m+c-1;

13     int t1=0;

14     LL b=x2;

15     while(b)

16     {

17         b/=10;

18         t1++;

19     }

20     int t2=0;

21     LL b1=m;

22     while(b1)

23     {

24         b1/=10;

25         t2++;

26     }

27     if(t1==t2)

28     {

29         ans+=c*t1*k;

30         if(ans<=w&&ans>=0) return true;

31         else return false;

32     }

33     LL x1=1;

34     int a1=t1-1;

35     while(a1--)

36     {

37         x1*=10;

38     }

39     ans+=(x2-x1+1)*t1*k;

40     LL x3=1;

41     int a2=t2;

42     while(a2--)

43     {

44         x3*=10;

45     }

46     ans+=(x3-m)*t2*k;

47     for(LL i=x3; i<x1; i*=10)

48     {

49         t2++;

50         ans+=(i*10-i)*t2*k;

51     }

52     if(ans<=w&&ans>=0) return true;

53     else return false;

54 

55 }

56 

57 int main()

58 {

59     while(scanf("%I64d%I64d%I64d",&w,&m,&k)!=EOF)

60     {

61         LL l=1,r=w;

62         LL ans=0;

63         while(l<=r)

64         {

65             LL mid=(l+r)/2;

66             if(ok(mid))

67             {

68                 ans=mid;

69                 l=mid+1;

70             }

71             else

72                 r=mid-1;

73         }

74         printf("%I64d\n",ans);

75     }

76     return 0;

77 }

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