乘法表

题目链接： http://exercise.acmcoder.com/online/online_judge_ques?ques_id=3819&konwledgeId=40

解题思路： 二分答案。

#include <bits/stdc++.h>
using namespace std;

const int MAXN = 100005;
typedef long long LL;

int in(LL x, int n,int m)
{
    int p=sqrt(x);
    for (int i=1;i<=p;++i)
    {
        if (x%i==0 && ((i<=n && (int)(x/i)<=m) || (i<=m && (int)(x/i)<=n))) return 1;
    }
    return 0;
}

LL cal(LL x, int n, int m)
{
    LL ans = 0LL;
    for (int i = 1;i<=m;++i)
        ans += min((LL)n*i,x)/i;
    return ans;
}

LL bisearch(LL l, LL r, LL k, int n, int m)
{
    LL mid;
    while(l<=r)
    {
        mid= l + (r-l)/2;
        // printf("l=%lld r=%lld mid=%lld
", l, r, mid);
        if (cal(mid, n, m)>=k) r=mid-1;
        else l=mid+1;
    }
    LL ans=r+1;
    while(!in(ans,n,m)) ++ans;
    return ans;
}


int main()
{
#ifndef ONLINE_JUDGE
    freopen("test.txt","r",stdin);
#endif // ONLINE_JUDGE
    int n,m;
    LL k;
    while (cin>>n>>m>>k)
    {
        if (n<m) swap(n,m);
        printf("%lld
",bisearch(1,(LL)n*m,k,n,m));
    }
    return 0;
}