在arrayList中移动

Question

public static <T> ArrayList<T> rotate(ArrayList<T> aL, int shift) 方法接受 String 的 Arraylist（至少在这个例子中）和一个 shift，它指示数组列表应该移动多少。如果我有，让我们说一个 arayList 的

[ A, B, C, D, E, F, G] 

而移位是2，所以方法返回

[ F, G, A, B, C, D, E]

或者另一个例子，

[ A, B, C, D, E, F, G]

而shift是4，则方法返回

[ D, E, F, G, A, B, C] 

我的方法完全错误，不知道如何解决这个问题。 smb 可以帮我吗？

import java.util.ArrayList;
public class Rotation{
  // Demonstrate rotat(a,shift) method
  public static void main(String arg[]){
    ArrayList<Character> charsL;
    charsL = new ArrayList<Character>();
    char [] chars = { 'A', 'B', 'C',
                      'D', 'E', 'F'};
    for(char c : chars){
      charsL.add(c);
    }
    // charsL = [ A, B, C, D, E, F]

    ArrayList<Character> result1;

    result1 = rotate(charsL, 2);
    // result1== [ E, F, A, B, C, D]
    System.out.println(result1);
    
    result1 = rotate(charsL, 7);
    // result1== [ F, A, B, C, D, E]
    System.out.println(result1);

    // WORKS WITH SRTINGS TOO
    ArrayList<String> stringL;
    stringL = new ArrayList<String>();
    String [] strs = { "A", "B", "C",
                       "D", "E", "F", "G" };
    for(String s : strs){
      stringL.add(s);
    }
    // stringL = [ A, B, C, D, E, F, G]

    ArrayList<String> result2;

    result2 = rotate(stringL, 7);
    // result2== [ A, B, C, D, E, F, G]
    System.out.println(result2);

    result2 = rotate(stringL, 4);
    // result2== [ D, E, F, G, A, B, C]
    System.out.println(result2);
  }

  public static <T>
  ArrayList<T> rotate(ArrayList<T> aL, int shift){
    // YOUR DEFINITION HERE 
      
      ArrayList <T> newValues = new ArrayList<>();
      ArrayList <T> temp = new ArrayList<>();
      
      for(int i = 0; i < aL.size(); i++)
      {
          newValues.remove(aL.get(shift));
          newValues.add(aL.get(i));
         //newValues.add(shift, aL.get(i));
          
      }
      return newValues;

  }

}

Answer 1

如果你可以使用内置的解决方案，那么你的代码可以很简单

public static <T> List<T> rotate(List<T> aL, int shift) {
    List<T> newValues = new ArrayList<>(aL);
    Collections.rotate(newValues, shift);
    return newValues;
}

Answer 2

试试这个：

public static <T> ArrayList<T> rotate(ArrayList<T> aL, int shift)
{
    if (aL.size() == 0)
        return aL;

    T element = null;
    for(int i = 0; i < shift; i++)
    {
        // remove last element, add it to front of the ArrayList
        element = aL.remove( aL.size() - 1 );
        aL.add(0, element);
    }

    return aL;
}

Answer 3

试试这个：

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class ALUtils {

public static <T> ArrayList<T> rotate(List<T> aL, int shift) {
    ArrayList<T> newValues = new ArrayList<>(aL);
    Collections.rotate(newValues, shift);
    return newValues;
}
}

测试用例：

// Public tests for ALUtils

import org.junit.*;
import static org.junit.Assert.*;
import java.util.*;
import java.io.*;

public class ALUtilsTests {
  /*Main method runs tests in this file*/ 
  public static void main(String args[]) {
    org.junit.runner.JUnitCore.main("Tests");
  } 

  public <T> void checkRotate(T d[], int shift, String expectS){
    ArrayList<T> a = new ArrayList<T>();
    a.addAll(Arrays.asList(d));
    String sourceBefore = a.toString();

    ArrayList<T> actualA = ALUtils.rotate(a, shift);

    String sourceAfter = a.toString();
    String actualS = actualA.toString();
    String msg =
      String.format("Rotation incorrect\n")+
      String.format("Source  : %s (before rotate)\n",sourceBefore)+
      String.format("Shift   : %d\n",shift)+
      String.format("Expect  : %s\n",expectS)+
      String.format("Actual  : %s\n",actualS)+
      String.format("Source  : %s (after rotate)\n",sourceAfter);
    assertEquals(msg,expectS,actualS);
    assertEquals(msg,sourceBefore,sourceAfter);
  }

  @Test public void test_rotate1() {
    Character [] d = { 'A', 'B', 'C', 'D', 'E', 'F'};
    int shift = 2;
    String expectS = "[E, F, A, B, C, D]";
    checkRotate(d,shift,expectS);
  }

  @Test public void test_rotate2() {
    Character [] d = { 'A', 'B', 'C', 'D', 'E', 'F'};
    int shift = 7;
    String expectS = "[F, A, B, C, D, E]";
    checkRotate(d,shift,expectS);
  }

  @Test public void test_rotate3() {
    String [] d =  { "A", "B", "C", "D", "E", "F", "G" };
    int shift = 7;
    String expectS = "[A, B, C, D, E, F, G]";
    checkRotate(d,shift,expectS);
  }

  @Test public void test_rotate4() {
    String [] d =  { "A", "B", "C", "D", "E", "F", "G" };
    int shift = 4;
    String expectS = "[D, E, F, G, A, B, C]";
    checkRotate(d,shift,expectS);
  }

  @Test public void test_rotate5() {
    Integer [] d =  { 1};
    int shift = 4;
    String expectS = "[1]";
    checkRotate(d,shift,expectS);
  }

  @Test public void test_rotate6() {
    Integer [] d =  {};
    int shift = 2;
    String expectS = "[]";
    checkRotate(d,shift,expectS);
  }

  @Test public void test_rotate7() {
    String [] d =  { "A", "B", "C", "D", "E", "F", "G" };
    int shift = 0;
    String expectS = "[A, B, C, D, E, F, G]";
    checkRotate(d,shift,expectS);
  }

  @Test public void test_rotate8() {
    String [] d =  { "A", "B", "C", "D", "E", "F", "G" };
    int shift = 28;
    String expectS = "[A, B, C, D, E, F, G]";
    checkRotate(d,shift,expectS);
  }

  @Test public void test_rotate9() {
    String [] d =  { "A", "B", "C", "D", "E", "F", "G" };
    int shift = 39;
    String expectS = "[D, E, F, G, A, B, C]";
    checkRotate(d,shift,expectS);
  }
}

Answer 4

目前还不清楚为什么要保留calling aL.get(shift)，以及为什么要不断从只应添加的列表中删除。

试试：

public static <T> ArrayList<T> rotate(ArrayList<T> l, int shift) {
    List<T> left = l.subList(0, aL.size() - shift);
    List<T> right = l.subList(aL.size() - shift, l.size());
    List<T> res = new ArrayList<T>(right);
    res.addAll(left);
    return res;
}