二分！！！

AC代码例如以下：

#include<iostream>

#include<cstring>

#include<cstdio>

#define ll long long

using namespace std;

ll n,m,k;

ll work(ll a)

{

    ll i,j;

    ll ans=0;

    for(i=1;i<=n;i++)

    {

        j=a/i;

        if(j>m)

            j=m;

        ans+=j;

    }

    return ans;

}

int main()

{

    scanf("%I64d%I64d%I64d",&n,&m,&k);

    ll l,r,mid;

    l=1;

    r=n*m+1;

    while(l<r)

    {

        mid=(l+r)/2;

        //cout<<mid<<"!!!!!!!!!!!!!!!!!!!!!!"<<work(mid)<<endl;

        if(work(mid)<k) l=mid+1;

        else r=mid;

    }

    cout<<r<<endl;

    return 0;

}