## Max Points on a Line

### 描述

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

### 以边为中心

// Max Points on a Line
// 暴力枚举法，以边为中心，时间复杂度O(n^3)，空间复杂度O(1)
class Solution {
public:
int maxPoints(vector<Point> &points) {
if (points.size() < 3) return points.size();
int result = 0;

for (int i = 0; i < points.size() - 1; i++) {
for (int j = i + 1; j < points.size(); j++) {
int sign = 0;
int a, b, c;
if (points[i].x == points[j].x) sign = 1;
else {
a = points[j].x - points[i].x;
b = points[j].y - points[i].y;
c = a * points[i].y - b * points[i].x;
}
int count = 0;
for (int k = 0; k < points.size(); k++) {
if ((0 == sign && a * points[k].y == c +  b * points[k].x) ||
(1 == sign&&points[k].x == points[j].x))
count++;
}
if (count > result) result = count;
}
}
return result;
}
};


### 以点为中心

// Max Points on a Line
// 暴力枚举，以点为中心，时间复杂度O(n^2)，空间复杂度O(n^2)
class Solution {
public:
int maxPoints(vector<Point> &points) {
if (points.size() < 3) return points.size();
int result = 0;

unordered_map<double, int> slope_count;
for (int i = 0; i < points.size()-1; i++) {
slope_count.clear();
int samePointNum = 0; // 与i重合的点
int point_max = 1;    // 和i共线的最大点数

for (int j = i + 1; j < points.size(); j++) {
double slope; // 斜率
if (points[i].x == points[j].x) {
slope = std::numeric_limits<double>::infinity();
if (points[i].y == points[j].y) {
++ samePointNum;
continue;
}
} else {
if (points[i].y == points[j].y) {
// 0.0 and -0.0 is the same
slope = 0.0;
} else {
slope = 1.0 * (points[i].y - points[j].y) /
(points[i].x - points[j].x);
}
}

int count = 0;
if (slope_count.find(slope) != slope_count.end())
count = ++slope_count[slope];
else {
count = 2;
slope_count[slope] = 2;
}

if (point_max < count) point_max = count;
}
result = max(result, point_max + samePointNum);
}
return result;
}
};