按字典顺序对点数组进行排序

Question

I am trying to solve an NX2 array of all integers in lexicographic order. I am not getting a way how to achieve it. Also, this should be done is as least time as possible. Here's the example.

Input

2 2
1 1
4 3
2 1
10 1

Output

1 1
2 1
2 2
4 3
10 1

Answer 1

1. Implement your own class Point with two fields of integers.
2. Make it implement Comparable (or implement a Comparator for this class).
3. Populate an array or List of these points, and sort it using the relevant sort: Arrays.sort() or Collections.sort() respectively.
4. Iterate the resulting array / List and extract back your points.

Answer 2

This is basically the same problem as sorting a bunch of integers, except that instead of integers you have a set of objects (in this case tuples - or in your code two item int arrays) that need to be sorted.

Unlike Python, Java does not automatically infer a lexicographical ordering for things like int[], so you can just pass it into a sort function and expect it to work. Fortunately, java allows you to define what the ordering should be and then use the built in sort method to sort it. This is done by implementing a Comparator and then defining its compare method that tells it how to compare two objects.

A (very simplified) example given below.

import java.util.*;

class Main{
        public static void main(String args[]){
                int[][] array={{2,2},{1,1},{4,3},{2,1},{10,1}};
                Arrays.sort(array, new Comparator<int[]>(){
                        public int compare(int[] a, int[] b){
                                //assumes array length is 2
                                int x,y;
                                if (a[0]!=b[0]) {
                                        x=a[0];y=b[0];
                                }
                                else{
                                        x=a[1];y=b[1];
                                }
                                if (x<y) return -1;
                                else if (x==y) return 0;
                                else return +1;
                        }
                });
                for(int[] term: array){
                        System.out.println(Arrays.toString(term));
                }
        }
}

Answer 3

Here try using this Point class with the comparator:

class MyPoint implements Comparable <MyPoint> {
    int x, y;
    public MyPoint(int x, int y) {
        this.x = x;
        this.y = y;
    }

    public int compareTo(MyPoint pt) {
        int cmp = Integer.compare(x, pt.x);
        if (cmp == 0) {
            return Integer.compare(y, pt.y);
        } else {
            return cmp;
        }
    }

}

After all you need to do is create a some Point objects and sort them. Here's an example:

    ArrayList<MyPoint> pts = new ArrayList<MyPoint>(4);
    pts.add( new MyPoint(3, 5));
    pts.add( new MyPoint(2, 3));
    pts.add( new MyPoint(3, 4));
    pts.add( new MyPoint(1, 5));
    Collections.sort(pts);
    for (int i = 0; i < pts.size(); i++) {
        MyPoint pt = pts.get(i);
        System.out.println(pt.x + " " + pt.y);
    }   

Output:

1 5
2 3
3 4
3 5

Answer 4

@amit explained what to do. I just wanted to add that you do not want to sort numeric data lexicographically. If you do this you get

1 1
10 1
2 1
etc.

Because lexicographically 10 is just after 1 and before 2. You want to perform regular numeric sort. Sometimes it is called "natural" sort.