链接列表排序不影响main（C ++）中的列表项？

Question

I am attempting to sort a linked list of points based on their distance to the origin. The closer the point, it should be the first/next in the linked list. Example: If a user inputs (2,2) (4,4) (1,1) (3,3) the linked list should be reordered using the next pointer to (1,1) (2,2) (3,3) (4,4). It works except that the points in main are not affected by the sorting and the points are not interchanged, so if (1,1) is 3rd like in the example it will be the smallest point the 2nd time around the loop again when compared to (4,4).

#include <iostream>
#include <math.h>
using namespace std;

class List {
public:
    int x;
    int y;
    List *next;
};
void init(List *root) {
    int x, y;
    List *traverse;
    traverse = root;
    while(traverse != 0) {
        cout<<"Enter coordinate x: ";
        cin>>x;
        traverse->x = x;
        cout<<"Enter coordinate y: ";
        cin>>y;
        traverse->y = y;
        traverse = traverse->next;
    }
}
void display(List *root) {
    List *traverse;
    traverse = root;
    while(traverse->next != 0) {
        cout<<"("<<traverse->x<<","<<traverse->y<<") ";
        traverse = traverse->next;
    }
    cout<<"("<<traverse->x<<","<<traverse->y<<")"<<endl;
}
void sort(List *root, int n) {
    List *traverse;
    List *stable;
    traverse = root;
    stable = root;
    double d1, d2;
    for(int i = 0; i < n; i++) {
        d1 = sqrt(pow(traverse->x, 2) + pow(traverse->y, 2));
        d2 = sqrt(pow(stable->x, 2) + pow(stable->y, 2));
        if(d1 < d2) {
            root = traverse;
        }
        traverse = traverse->next;
    }
}
void collinear(List *root) {
    int x[3] = {0}, y[3] = {0};
    int value;
    List *traverse;
    traverse = root;
    for(int i = 0; i < 3; i++) {
        if(traverse != 0) {
            x[i] = traverse->x;
            y[i] = traverse->y;
            traverse = traverse->next;
        }
    }
    if(x[2] != 0) {
        value = x[0] * (y[1] - y[2]) + x[1] * (y[2] - y[0]) + x[2] * (y[0] - y[1]);
        if(value == 0) {
            traverse = root;
            for(int i = 0; i < 3; i++) {
                cout<<"("<<traverse->x<<","<<traverse->y<<") ";
                traverse = traverse->next;
            }
            cout<<"collinear!"<<endl;
        }
        else {
            traverse = root;
            for(int i = 0; i < 3; i++) {
                cout<<"("<<traverse->x<<","<<traverse->y<<") ";
                traverse = traverse->next;
            }
            cout<<"non-collinear!"<<endl;
        }
    }
    else {
        cout<<"Not a group of 3 points cannot calculate collinearity!"<<endl;
    }
}
int main() {
    List *root;
    root = new List;
    List *traverse;
    traverse = root;
    List *node1;
    node1 = new List;
    List *node2;
    node2 = new List;
    List *node3;
    node3 = new List;
    root->next = node1;
    node1->next = node2;
    node2->next = node3;
    node3->next = 0;
    init(traverse);
    display(traverse);
    /*for(int i = 0; i < 2; i++) {
        collinear(traverse);
        traverse = traverse->next;
    }
    traverse = root;*/
    traverse = root;
    int n = 4;
    for(int i = 0; i < 4; i++) {
        List *traverse2;
        traverse2 = traverse;
        sort(traverse2, n);
        traverse = traverse->next;
        n--;
    }
    traverse = root;
    display(traverse);
    return 0;
}

Answer 1

If you want to be able to change the root you'll need to pass it as a **. However even then I dont think your sort function will work it will just return a truncated list missing everything before the smallest element.

Is their any reason your using a custom linked list and sort alogorithm? The standard library takes care of things like containers and sorting:

#include <vector>
#include <algorithm>
#include <iostream>

class Point
{
public:
    Point(int x_, int y_) :
        x(x_),
        y(y_)
    {
    }

    // no need to square root to compare distance from origin
    int distanceSquared() const
    {
        return (x*x)+(y*y);
    }

    // define an operator< so we can sort containers holding this class
    bool operator< ( const Point& rhs ) const
    {
        return distanceSquared() < rhs.distanceSquared();
    }

    // declare a friend function for outputing the values in a user friendly way
    friend std::ostream& operator<<(std::ostream& os, const Point& p);

    int x;
    int y;    
};

// and define the friend function outside the class
std::ostream& operator<<(std::ostream& os, const Point& p)
{
    os << "(" << p.x << ", " << p.y << ")";
    return os;
}

int main()
{
    // std::vector to store user entered points
    std::vector<Point> points;
    // temp storage for user input 
    int x, y;
    // entering anything non-numberic will exit this loop
    while( true )
    {
        std::cout << "Enter X coordinate : ";
        std::cin >> x;
        if( std::cin.fail() )
            break;
        std::cout << "Enter Y coordinate : ";
        std::cin >> y;
        if( std::cin.fail() )
            break;
        // we've read in a valid x,y so add a new point to vector
        points.push_back( Point(x, y) );
    }

    // sort will use the operator< function in Point class
    std::sort(points.begin(), points.end());   

    // use the stream operator to debug out our point
    for( auto it = points.begin(); it != points.end(); ++it )
    {
        std::cout << *it << std::endl;
    }
}