587. Erect the Fence

題意：輸出把所有點包圍起來的輪廓線上的點

我的思路：凸包問題，Graham-Scan算法

我的代碼：

class Solution {
public:
    static bool cmp(Point& a, Point& b) {
        if (a.y == b.y) return a.x < b.x;
        return a.y < b.y;
    }
    bool mult(Point sp, Point ep, Point op) {
        return (sp.x-op.x)*(ep.y-op.y) > (ep.x-op.x)*(sp.y-op.y);
    }
    vector
 
<Point> outerTrees(vector<Point>& points) {
        int n = points.size();
        if (n <= 3) return points;
        sort(points.begin(), points.end(), cmp);
        vector<Point> ans;
        ans.push_back(points[0]);
        ans.push_back(points[1]);
        for (int i = 2
 
; i < n; i++) {
            while (ans.size() > 1 && mult(points[i], ans[ans.size()-1], ans[ans.size()-2])) ans.pop_back();
            ans.push_back(points[i]);
        }
        int len = ans.size()-1;
        ans.push_back(points[n-2]);
        for (int i = n-3; i >= 0; i--) {
            
 
while (ans.size()-1 != len && mult(points[i], ans[ans.size()-1], ans[ans.size()-2])) ans.pop_back();
            ans.push_back(points[i]);
        }
        ans.pop_back();
        return ans;
    }
};

一刷leetcode——計算幾何