题意

Given n points on a 2D plane, find the maximum number of points that lie on the same straight line.

意思就是说在给定的节点中计算出在同一条直线上的最大节点个数。

思路

这道题，题意很容易理解，但是需要注意的东西包括，如果你用斜率计算，那么就需要注意到精确度的问题，否则就会变成相等的斜率，无奈之下，只能提高精确度，比如说用long double，但这不是持久的办法，目前的办法是使用最大公约数。

其实我大概的思路已经想到了，就是第五种方法的思路，这道题没做出来的原因是，没有考虑到相同点以及相同的x的情况。

代码

#include <stdio.h>
#include <stdio.h>
#include <string>
#include <vector>
#include <iostream>
#include <unordered_map>
#include <map>
#include <set>
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) {}
};/***  第一种方法：将每一个顶点和其他的顶点进行计算得出斜率，*  用hash保存下来，key为斜率，value为总个数*  因为相同斜率的则在同一条线上**  @param points <#points description#>**  @return <#return value description#>*/
int maxPoints(vector<Point>& points) {unordered_map<int, int> hash;int maxCnt = 0;auto caculateK = [&](Point p1, Point p2) {int k = (p2.y - p1.y)/(p2.x - p1.x);return k;};for (int i = 0; i < points.size(); i++) {Point pointA = points[i];for (int j = 0; j < points.size() && i != j; j++) {Point pointB = points[j];int k = caculateK(pointA, pointB);if (hash.find(k) != hash.end()) {hash[k]++;}else {hash.insert(make_pair(k, 1));}}//        hash.clear();}for (auto itr = hash.begin(); itr != hash.end(); itr++) {maxCnt = std::max((*itr).second, maxCnt);}return maxCnt + 2;
}/***  计算最大公约数*/
int GCD(int a, int b) {if(b==0) return a;else return GCD(b, a%b);
}/***  这个方法不再是使用斜率作为map的key，而是使用的是gcd(最大公约数)*  总之结果就是overlap＋localmax＋vertical＋1，*  其中overlap代表的是完全相同的顶点，vertical代表的是只有x是相等的，因为相减会导致0**  @param points <#points description#>**  @return <#return value description#>*/
int maxPoints2(vector<Point>& points) {if (points.size() < 2) {return (int)points.size();}int result = 0;for (int i = 0; i < points.size(); i++) {map<pair<int, int>, int> lines;int localmax = 0, overlap = 0, vertical = 0;for (int j = i+1; j < points.size(); j++) {// 相同的点的个数if (points[j].x == points[i].x && points[j].y == points[i].y) {overlap++;continue;}else if (points[j].x == points[i].x) {vertical++;}else {int a=points[j].x-points[i].x, b=points[j].y-points[i].y;int gcd=GCD(a, b);a/=gcd;b/=gcd;lines[make_pair(a, b)]++;localmax=std::max(lines[make_pair(a, b)], localmax);}localmax=std::max(vertical, localmax);}// 结果为result = std::max(result, localmax+overlap+1); //这里加一是因为之前vertical只计算了一次}return result;
}// 由于精确度问题，更改double
inline bool double_equal(double a, double b) { return abs(a-b) < 1e-10; }
inline bool double_less (double a, double b) { return a-b < -1e-10; }struct Line {double r;  // ratio ; slopedouble t;  // translationLine(Point p, Point q) { // mathif (q.x == p.x) r = 1e20, t = p.x;else{r = (double) (q.y-p.y) / (double) (q.x-p.x);t = p.y - p.x * r;}}
};// 用作排序
bool operator < (const Line& a, const Line& b) {return a.r == b.r ? a.t < b.t : a.r < b.r;
}// 依此来判断map中是否相等
bool operator == (const Line& a, const Line& b) {return a.r == b.r && a.t == b.t;
}/***  这个思路最大的特点就是利用了一个结构体和set的原理*  因为set可以自动去除掉重复的元素**  @param points <#points description#>**  @return <#return value description#>*/
int maxPoints4(vector<Point> &points) {if (points.empty()) return 0;map<Line, set<Point*> > line_map;for (auto & a : points)for (auto & b : points){Line line(a,b);line_map[line].insert(&a);line_map[line].insert(&b);}int ret = 1;for (auto & pr : line_map) ret = max(ret,(int)pr.second.size());return ret;
}/***  最直接最简单的做法**  @param points <#points description#>**  @return <#return value description#>*/
int maxPoints5(vector<Point>& points) {if(points.empty())return 0;else if(points.size() == 1)return 1;int ret = 0;for(int i = 0; i < points.size(); i ++){//start pointint curmax = 1; //points[i] itselfunordered_map<long double, int> kcnt;    // slope_k countint vcnt = 0;   // vertical countint dup = 0;    // duplicate added to curmaxfor(int j = 0; j < points.size(); j ++){if(j != i){long double deltax = points[i].x - points[j].x;long double deltay = points[i].y - points[j].y;if(deltax == 0 && deltay == 0) {dup ++;}else if(deltax == 0){if(vcnt == 0)vcnt = 2;elsevcnt ++;curmax = max(curmax, vcnt);}else{long double k = deltay / deltax;if(kcnt[k] == 0)kcnt[k] = 2;elsekcnt[k] ++;curmax = max(curmax, kcnt[k]);}}}ret = max(ret, curmax + dup);}return ret;
}int main(int argc, const char * argv[]) {vector<Point> points;Point point1(0, 0), point2(94911151, 94911150), point3(94911152, 94911151), point4(5, 6);points.push_back(point1);points.push_back(point2);points.push_back(point3);int result = maxPoints5(points);cout << "result..." << result << endl;return 0;
}