poj3685

是关于 i 的递增函数。

#include <stdio.h>
#include <string.h>
#include <algorithm>
using namespace std;

#define rep(i,s,t) for(int i=s;i<t;i++)
typedef long long ll;

int t,n;
ll m;

inline ll Cal(int i,int j){
    return i*1LL*i+100000*1LL*i+j*1LL*j-100000*1LL*j+i*1LL*j;
}

inline ll Sum(ll val){
    ll num=0;
    rep(i,1,n+1){
        int s=1,t=n;
        while(s<=t){
            int mid=(s+t)>>1;
            ll v=Cal(mid,i);
            if(v<=val) s=mid+1;
            else t=mid-1;
        }
        num+=s-1;
    }
    return num;
}

int main(){
    scanf("%d",&t);
    while(t--){
        scanf("%d%lld",&n,&m);
        ll l=-1000000000000*1LL,r=1000000000000*1LL;
        while(l<=r){
            //printf("%lld %lld ",l,r);
            ll mid=l+(r-l)/2;
            ll v=Sum(mid);
            if(v<m) l=mid+1;
            else r=mid-1;
            //  printf("%lld %lld %lld %lld \n",l,r,mid,v); 
        }
        printf("%lld\n",l);
    }
    return 0;
}