POJ 3179 Corral the Cows

#include<cstdio>
#include<algorithm>
using namespace std;
const int maxn = 510;
int C,n;
struct node{
    int x,y;
}p[maxn];
int xn,yn,rx[maxn],ry[maxn];
int sum[maxn][maxn];
bool cmp1(node a,node b){
    return a.x<b.x;
}
bool cmp2(node a,node b){
    return a.y<b.y;
}
bool check(int k){
    for(int a=1,b=1;;a++){
        while(rx[b+1]-rx[a]+1<=k&&b<xn) b++;
        for(int c=1,d=1;;c++){
            while(ry[d+1]-ry[c]+1<=k&&d<yn) d++;
            int ans=sum[b][d]+sum[a-1][c-1]-sum[a-1][d]-sum[b][c-1];
            if(ans>=C) return true;
            if(d==yn) break;
        }
        if(b==xn) break;
    }
    return false;
}

int main(){
    scanf("%d%d",&C,&n);
    for(int i=1;i<=n;i++){
        scanf("%d%d",&p[i].x,&p[i].y);
    }
    sort(p+1,p+n+1,cmp1);
    xn=1;rx[1]=p[1].x;p[1].x=1;
    for(int i=2;i<=n;i++){
        if(p[i].x!=p[i-1].x) rx[++xn]=p[i].x;
        p[i].x=xn;
    }
    sort(p+1,p+n+1,cmp2);
    yn=1;ry[1]=p[1].y;p[1].y=1;
    for(int i=2;i<=n;i++){
        if(p[i].y!=p[i-1].y) ry[++yn]=p[i].y;
        p[i].y=yn;
    }
    for(int i=1;i<=n;i++) sum[p[i].x][p[i].y]++;
    for(int i=1;i<=xn;i++){
        for(int j=1;j<=yn;j++){
            sum[i][j]+=sum[i][j-1]+sum[i-1][j]-sum[i-1][j-1];
        }
    }
    int l=1,r=max(rx[xn],ry[yn]),ans;
    while(l<=r){
        int mid=l+r>>1;
        if(check(mid)){
            ans=mid;
            r=mid-1;
        }else{
            l=mid+1;
        }
    }
    printf("%d\n",ans);
}