# Count maximum points on same line

Given N point on a 2D plane as pair of (x, y) co-ordinates, we need to find maximum number of point which lie on the same line.

Examples:

Input : points[] = {-1, 1}, {0, 0}, {1, 1}, {2, 2}, {3, 3}, {3, 4} Output : 4 Then maximum number of point which lie on same line are 4, those point are {0, 0}, {1, 1}, {2, 2}, {3, 3}

We can solve above problem by following approach â€“ For each point p, calculate its slope with other points and use a map to record how many points have same slope, by which we can find out how many points are on same line with p as their one point. For each point keep doing the same thing and update the maximum number of point count found so far.

Some things to note in implementation are:

1) if two point are (x1, y1) and (x2, y2) then their slope will be (y2 â€“ y1) / (x2 â€“ x1) which can be a double value and can cause precision problems. To get rid of the precision problems, we treat slope as pair ((y2 – y1), (x2 â€“ x1)) instead of ratio and reduce pair by their gcd before inserting into map. In below code points which are vertical or repeated are treated separately.

2) If we use unordered_map in c++ or HashMap in Java for storing the slope pair, then total time complexity of solution will be O(n^2)

## C++

`/* C/C++ program to find maximum number of point` `which lie on same line */` `#include <bits/stdc++.h>` `#include <boost/functional/hash.hpp>` `using` `namespace` `std;` `// method to find maximum collinear point` `int` `maxPointOnSameLine(vector< pair<` `int` `, ` `int` `> > points)` `{` ` ` `int` `N = points.size();` ` ` `if` `(N < 2)` ` ` `return` `N;` ` ` `int` `maxPoint = 0;` ` ` `int` `curMax, overlapPoints, verticalPoints;` ` ` `// here since we are using unordered_map` ` ` `// which is based on hash function` ` ` `//But by default we don't have hash function for pairs` ` ` `//so we'll use hash function defined in Boost library` ` ` `unordered_map<pair<` `int` `, ` `int` `>, ` `int` `,boost::` ` ` `hash<pair<` `int` `, ` `int` `> > > slopeMap;` ` ` `// looping for each point` ` ` `for` `(` `int` `i = 0; i < N; i++)` ` ` `{` ` ` `curMax = overlapPoints = verticalPoints = 0;` ` ` `// looping from i + 1 to ignore same pair again` ` ` `for` `(` `int` `j = i + 1; j < N; j++)` ` ` `{` ` ` `// If both point are equal then just` ` ` `// increase overlapPoint count` ` ` `if` `(points[i] == points[j])` ` ` `overlapPoints++;` ` ` `// If x co-ordinate is same, then both` ` ` `// point are vertical to each other` ` ` `else` `if` `(points[i].first == points[j].first)` ` ` `verticalPoints++;` ` ` `else` ` ` `{` ` ` `int` `yDif = points[j].second - points[i].second;` ` ` `int` `xDif = points[j].first - points[i].first;` ` ` `int` `g = __gcd(xDif, yDif);` ` ` `// reducing the difference by their gcd` ` ` `yDif /= g;` ` ` `xDif /= g;` ` ` `// increasing the frequency of current slope` ` ` `// in map` ` ` `slopeMap[make_pair(yDif, xDif)]++;` ` ` `curMax = max(curMax, slopeMap[make_pair(yDif, xDif)]);` ` ` `}` ` ` `curMax = max(curMax, verticalPoints);` ` ` `}` ` ` `// updating global maximum by current point's maximum` ` ` `maxPoint = max(maxPoint, curMax + overlapPoints + 1);` ` ` `// printf("maximum collinear point` ` ` `// which contains current point` ` ` `// are : %d\n", curMax + overlapPoints + 1);` ` ` `slopeMap.clear();` ` ` `}` ` ` `return` `maxPoint;` `}` `// Driver code` `int` `main()` `{` ` ` `const` `int` `N = 6;` ` ` `int` `arr[N][2] = {{-1, 1}, {0, 0}, {1, 1}, {2, 2},` ` ` `{3, 3}, {3, 4}};` ` ` `vector< pair<` `int` `, ` `int` `> > points;` ` ` `for` `(` `int` `i = 0; i < N; i++)` ` ` `points.push_back(make_pair(arr[i][0], arr[i][1]));` ` ` `cout << maxPointOnSameLine(points) << endl;` ` ` `return` `0;` `}` |

## Python3

`def` `maxPoints(points):` ` ` `n` `=` `len` `(points)` ` ` `# upto two points all points will be part of the line` ` ` `if` `n<` `3` `:` ` ` `return` `n` ` ` `max_val` `=` `0` ` ` `# looping for each point` ` ` `for` `i ` `in` `points:` ` ` `# Creating a dictionary for every new` ` ` `# point to save memory` ` ` `d ` `=` `{}` ` ` `dups ` `=` `0` ` ` `cur_max ` `=` `0` ` ` `# pairing with all other points` ` ` `for` `j ` `in` `points:` ` ` `if` `i!` `=` `j:` ` ` `if` `j[` `0` `]` `=` `=` `i[` `0` `]: ` `#vertical line` ` ` `slope` `=` `'inf'` ` ` `else` `:` ` ` `slope` `=` `float` `(j[` `1` `]` `-` `i[` `1` `])` `/` `float` `(j[` `0` `]` `-` `i[` `0` `])` ` ` `# Increasing the frequency of slope and` ` ` `# updating cur_max for current point(i)` ` ` `d[slope] ` `=` `d.get(slope,` `0` `)` `+` `1` ` ` `cur_max` `=` `max` `(cur_max, d[slope])` ` ` `# if both points are equal same increase` ` ` `# duplicates count.` ` ` `# Please note that this will also increment` ` ` `# when we map it with itself.` ` ` `# we still do it because we will not have to` ` ` `# add the extra one at the end.` ` ` `else` `:` ` ` `dups` `+` `=` `1` ` ` `max_val` `=` `max` `(max_val, cur_max` `+` `dups)` ` ` `return` `max_val` `# Driver code` `points ` `=` `[(` `-` `1` `, ` `1` `), (` `0` `, ` `0` `), (` `1` `, ` `1` `), (` `2` `, ` `2` `), (` `3` `, ` `3` `), (` `3` `, ` `4` `)]` `print` `(maxPoints(points))` |

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