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
",ans);
75     }
76     return 0;
77 }
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