• ``````class Solution {
private:
static bool equal(const Point & a, const Point & b) {
return (a.x == b.x && a.y == b.y);
}
static double slope(const Point & a, const Point & b) {
if (equal(a, b))
return -std::numeric_limits<double>::infinity();
else if (a.x == b.x)
return std::numeric_limits<double>::infinity();
else if (a.y == b.y)
return 0.0;
else
return (a.y - b.y) / (double) (a.x - b.x);
}
struct Less_slope {
Point ref;
Less_slope(Point & p) {
ref = Point(p.x, p.y);
}
bool operator() (const Point & p1, const Point & p2) {
return slope(ref, p1) < slope(ref, p2);
}
};

public:
int maxPoints(vector<Point> &points) {
if (points.size() < 2) {
return (int)points.size();
}
int max_num = 2;
for (int i = 0; i < points.size() - max_num; i++)
{
int duplicate = 0;
int local_max = 2;
std::sort(points.begin()+i+1, points.end(), Less_slope(points[i]));
for(int j = i+1; j < points.size()-1; j++)
{
if (equal(points[i], points[j])) {
//duplicate points
duplicate ++;
continue;
}

double slope1 = slope(points[i], points[j]);
double slope2 = slope(points[i], points[j+1]);

if (slope2 != slope1) {
//Not on the same line
continue;
}

int count = 3;
j += 2;
while (j < points.size() && slope(points[i], points[j]) == slope1) {
j++;
count ++;
}
j--;
if (count > local_max) local_max = count;
}

local_max += duplicate;
if (local_max > max_num) max_num = local_max;
}

return max_num;
}
};
``````

I failed to pass the following test on OJ:

[(84,250),(0,0),(1,0),(0,-70),(0,-70),(1,-1),(21,10),(42,90),(-42,-230)]

It said that my output was 3 but the correct answer is 6.

However, when I tested on my laptop, the output was exactly 6. Here are my codes for testing:

``````#include <cstdio>
#include <algorithm>
#include <limits>
#include <vector>
using namespace std;

//Definition for a point.
struct Point {
int x;
int y;
Point() : x(0), y(0) {}
Point(int a, int b) : x(a), y(b) {}
};

class Solution {...};

int main()
{
vector<Point> input;
int n;
scanf("%d", &n);

int x, y;
while (n--) {
scanf("%d", &x);
scanf("%d", &y);
input.push_back(Point(x,y));
}

printf("%d\n", Solution().maxPoints(input));

return 0;
}
``````

The input format is:

9

84 250

0 0

1 0

0 -70

0 -70

1 -1

21 10

42 90

-42 -230

I ran the codes on ideone.com as well, and the output was also:

6

Can anyone tell me why is the output on OJ different from the output of my own testing?

• This post is deleted!

• same here, run the things local can get right result

• Here are the two places I found in your code which compares doubles directly:

1. if (slope2 != slope1)
2. slope(points[i], points[j]) == slope1

You should never compare two doubles directly as that would lead to different precision error depending on machines and operating systems.

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