codeforces 817C Really Big Numbers 二分

传送门

题意：给一个n和s  n上个位十位~~到最高位 每一位相加得到sum 然后用这个n-sum>=s 称为大数。 比如10 个位加十位为1 10-1=9.

题解：10-19  相减必为9  20-29 相减必为18 ·····  所以有规律  可以用规律做。

但用二分简单粗暴一点。

#include <iostream>
#include <stdio.h>
#include <cstring>
#include <string>
#include <algorithm>
#include <queue>
#include <stack>
#include <cmath>
#include <map>
#include <bitset>
#include <set>
#include <vector>
#include <functional>
using namespace std;

#define pi acos(-1)
#define endl '\n'
#define rand() srand(time(0));
#define me(x) memset(x,0,sizeof(x));
#define foreach(it,a) for(__typeof((a).begin()) it=(a).begin();it!=(a).end();it++)
#define close() ios::sync_with_stdio(0);

typedef long long LL;
const int INF=0x3f3f3f3f;
const LL LINF=0x3f3f3f3f3f3f3f3fLL;
//const int dx[]={-1,0,1,0,-1,-1,1,1};
//const int dy[]={0,1,0,-1,1,-1,1,-1};
const int maxn=1e3+5;
const int maxx=1e5+100;
const double EPS=1e-7;
const int MOD=10000007;
#define mod(x) ((x)%MOD);
template<class T>inline T min(T a,T b,T c) { return min(min(a,b),c);}
template<class T>inline T max(T a,T b,T c) { return max(max(a,b),c);}
template<class T>inline T min(T a,T b,T c,T d) { return min(min(a,b),min(c,d));}
template<class T>inline T max(T a,T b,T c,T d) { return max(max(a,b),max(c,d));}
//typedef tree<pt,null_type,less< pt >,rb_tree_tag,tree_order_statistics_node_update> rbtree;
/*lch[root] = build(L1,p-1,L2+1,L2+cnt);
rch[root] = build(p+1,R1,L2+cnt+1,R2);中前*/
/*lch[root] = build(L1,p-1,L2,L2+cnt-1);
rch[root] = build(p+1,R1,L2+cnt,R2-1);中后*/
long long gcd(long long a , long long b){if(b==0) return a;a%=b;return gcd(b,a);}

LL solve(LL n)
{
LL ans = 0;
while(n)
{
ans += n%10;
n/=10;
}
return ans;
}
int main()
{
close();
LL n,s;
cin>>n>>s;
if(n<s)
{
puts("0");
return 0;
}
LL l=1,r=n+1;
while(l<=r)
{
LL mid=(l+r)>>1;
if(mid-solve(mid)<s) l=mid+1;
else r=mid-1;
}
LL ans=n-((l+r)>>1);
if(ans<=0)
ans=0;
cout<<ans<<endl;
}