Java算法-使用ArrayList合并排序

Question

I'm an AP Computer Science student and my current assignment is that I have to make a program that takes an ArrayList of numbers and, only using the standard Java API, sort it using a Merge Sort. There aren't any compiling errors, but on run-time it doesn't even return the ArrayList! After a little debugging I found that it isn't populating the original list. Please help! Code:

import java.io.*;
import java.util.*;

public class MergeSort {
    public static void main(String[] args) throws IOException{
    Scanner in  = new Scanner(System.in);
    Random r = new Random();
    int size, largestInt, holder;

    System.out.println("How many integers would you like me to create?");
    size = in.nextInt();
    ArrayList<Integer>list = new ArrayList<Integer>(size);
    System.out.println("What would the largest integer be?");
    largestInt = in.nextInt();

    for(int i = 0; i < list.size(); i++){
        holder = r.nextInt(largestInt + 1);
        list.add(holder);
    }
    mergeSort(list);

    for (int j = 0; j < list.size(); j++) {
        if(j == 19 || j == 39 || j == 59 || j == 79 || j == 99 || j == 119 || j == 139 || j == 159 || j == 179 || j == 199){
            System.out.print(list.get(j));
            System.out.println();
        }
        else{
            System.out.println(list.get(j) + "\t");
        }
    }

}

static void mergeSort(ArrayList<Integer> list) {
    if (list.size() > 1) {
        int q = list.size()/2;
        ArrayList<Integer> leftList = new ArrayList<Integer>();
        for(int i = 0; i > 0 && i <= q; i++){
            leftList.add(list.get(i));
        }
        ArrayList<Integer> rightList = new ArrayList<Integer>();
        for(int j = 0; j > q && j < list.size(); j++){
            rightList.add(list.get(j));
        }

        mergeSort(leftList);
        mergeSort(rightList);
        merge(list,leftList,rightList);
    }
}

static void merge(ArrayList<Integer> a, ArrayList<Integer> l, ArrayList<Integer> r) {
    int totElem = l.size() + r.size();
    int i,li,ri;
    i = li = ri = 0;
    while ( i < totElem) {
        if ((li < l.size()) && (ri<r.size())) {
            if (l.get(li) < r.get(ri)) {
                a.set(i, l.get(li));
                i++;
                li++;
            }
            else {
                a.set(i, r.get(ri));
                i++;
                ri++;
            }
        }
        else {
            if (li >= l.size()) {
                while (ri < r.size()) {
                    a.set(i, r.get(ri));
                    i++;
                    ri++;
                }
            }
            if (ri >= r.size()) {
                while (li < l.size()) {
                    a.set(i, l.get(li));
                    li++;
                    i++;
                }
            }
        }
    }
}

Answer 1

This is because list.size() returns 0 for an empty list. In your loop where you populate the list, replace list.size() with size .

I haven't checked the actual mergeSort part of your program, but the change that I suggested will at least make the initial population of the list work.