interviews/training->training

puzzles/training/max_points_on_a_line.cpp

@@ -0,0 +1,116 @@
+/*
+VIM: let b:lcppflags="-std=c++14 -O2 -pthread -I."
+VIM: let b:wcppflags="/O2 /EHsc /DWIN32"
+*/
+#include "stdafx.h"
+#include <iostream>
+#include <exception>
+#include <string>
+#include <chrono>
+#include <vector>
+#include <algorithm>
+#include <queue>
+#include <mutex>
+#include <condition_variable>
+#include <atomic>
+#include <unordered_map>
+#include <unordered_set>
+
+/*
+https://leetcode.com/problems/max-points-on-a-line/
+
+Given n points on a 2D plane, find the maximum number of points that lie on the same straight line.
+*/
+
+struct Point {
+	int x;
+	int y;
+	Point() : x(0), y(0) {}
+	Point(int a, int b) : x(a), y(b) {}
+};
+
+struct pairhash {
+	std::size_t operator() (const Point& p) const {
+		return std::hash<int>()(p.x) ^ std::hash<int>()(p.y);
+	}
+};
+
+bool operator == (const Point& p1, const Point& p2) {
+	return p1.x == p2.x && p1.y == p2.y;
+}
+
+Point ratio(int x, int y) {
+	if (x == 0) {
+		return Point(0, 1);
+	}
+	else if (y == 0) {
+		return Point(1, 0);
+	}
+	else if ( x < 0 ) {
+		x = -x;
+		y = -y;
+	}
+
+	int a = std::max(abs(x), abs(y));
+	int b = std::min(abs(x), abs(y));
+	for (int c; (c = a % b); a = b, b = c);
+	return Point(x / b, y / b);
+}
+
+int solve(const std::vector<Point>& v) {
+
+	std::unordered_map<Point, int, pairhash> vv;
+	for (const auto& p : v) {
+		vv[p]++;
+	}
+
+	if (vv.empty()) {
+		return 0;
+	}
+	else if (vv.size()==1) {
+		return vv.begin()->second;
+	}
+
+	int max_points = 1;
+	for (auto i = vv.begin(); i != vv.end(); i++) {
+		std::unordered_map<Point, int, pairhash> s;
+		for (auto j = std::next(i); j != vv.end(); j++) {
+			const auto line = ratio(i->first.x - j->first.x, i->first.y - j->first.y);
+			s[line] += j->second;
+			max_points = std::max(max_points, s[line] + i->second);
+		}
+	}
+
+	return max_points;
+}
+
+int main()
+{
+	try {
+		auto begin = std::chrono::high_resolution_clock::now();
+
+		std::cout << solve({ { 1,1 },{ 2,2 },{ 3,3 } }) << std::endl;
+		std::cout << solve({ { 1,1 },{ 3,2 },{ 5,3 },{ 4,1 },{ 2,3 },{ 1,4 } }) << std::endl;
+		std::cout << solve({ { 2, 3 },{ 3, 3 },{ -5, 3 } }) << std::endl;
+		std::cout << solve({ { 2, 2 },{ 3, 3 },{ 2, 2 },{ 3, 3 } }) << std::endl;
+		std::cout << solve({ { 2, 2 },{ 2, 3 },{ 3, 2 },{ 3, 3 } }) << std::endl;
+
+
+		auto end = std::chrono::high_resolution_clock::now();
+		std::chrono::duration<double> seconds = end - begin;
+		std::cout << "Time: " << seconds.count() << std::endl;
+		return 0;
+	}
+	catch (const std::exception& e)
+	{
+		std::cerr << std::endl
+			<< "std::exception(\"" << e.what() << "\")." << std::endl;
+		return 2;
+	}
+	catch (...)
+	{
+		std::cerr << std::endl
+			<< "unknown exception." << std::endl;
+		return 1;
+	}
+}