poj 1054（注意max_num的初值）

#include<iostream>
#include<algorithm>
using namespace std;
#define maxn 5005
int r,c,n;
typedef struct node{
    int x,y;
}Point;
Point point[maxn];
int cmp(Point p1,Point p2){
    if(p1.x<p2.x||(p1.x==p2.x&&p1.y<p2.y))return true;
    return false;
}
bool isIn(int x,int y){
    if(x<=0||x>r||y<=0||y>c)return false;
    return true;
}
bool search(int x,int y){
    int l=0,r=n,mid;
    while(l<=r){
        mid = (l+r)/2;
        if(x==point[mid].x&&y==point[mid].y)return true;
        else if(x<point[mid].x||(x==point[mid].x&&y<point[mid].y))r = mid-1;
        else l = mid+1; 
    }
    return false;
}
int main(){
    scanf("%d%d%d",&r,&c,&n);
    for(int i=0;i<n;i++){
        scanf("%d%d",&point[i].x,&point[i].y);
    }
    sort(point,point+n,cmp);
    int sum = 0;
    int max_num = 0;
    for(int i=0;i<n;i++){
        for(int j=i+1;j<n;j++){
            int x = point[j].x-point[i].x;
            int y = point[j].y-point[i].y;
            if(isIn(point[i].x-x,point[i].y-y)||
              !isIn(point[j].x+max_num*x,point[j].y+max_num*y)){
                  continue;
            }
            int num = 0;
            for(int k=1;isIn(point[j].x+k*x,point[j].y+k*y);k++){
                if(search(point[j].x+k*x,point[j].y+k*y))num++;
                else{
                    num = 0;
                    break;
                }
            }
            max_num = max(max_num,num);
        }
    }
    if(max_num)max_num+=2;
    printf("%d\n",max_num);
    return 0;
}