归并排序面试题——逆序对

递归方式和非递归方式实现归并排序——归并排序，递归和非递归实现

package com.harrison.class03;

/**
 * @author Harrison
 * @create 2022-02-24-17:03
 * @motto 众里寻他千百度，蓦然回首，那人却在灯火阑珊处。
 */
public class Code03_ReversePair {
    public static int reverPairNumber(int[] arr) {
        if (arr == null || arr.length < 2) {
            return 0;
        }
        return process(arr, 0, arr.length - 1);
    }

    // arr[L..R]既要排好序，也要求逆序对数量返回
    // 所有merge时，产生的逆序对数量，累加，返回
    // 左 排序 merge并产生逆序对数量
    // 右 排序 merge并产生逆序对数量
    public static int process(int[] arr, int L, int R) {
        if (L == R) {
            return 0;
        }
        int M = L + ((R - L) >> 1);
        return process(arr, L, M) + process(arr, M + 1, R) + merge(arr, L, M, R);
    }

    public static int merge(int[] arr,int L,int M,int R){
        int[] help=new int[R-L+1];
        int i=help.length-1;
        int p1=M;
        int p2=R;
        int res=0;
        while(p1>=L && p2>M){
            res+=arr[p1]>arr[p2]?(p2-M):0;
            help[i--]=arr[p1]>arr[p2]?arr[p1--]:arr[p2--];
        }
        while(p1>=L){
            help[i--]=arr[p1--];
        }
        while(p2>M){
            help[i--]=arr[p2--];
        }
        for(i=0; i<help.length; i++){
            arr[L+i]=help[i];
        }
        return res;
    }

    public static int comparator(int[] arr){
        int ans=0;
        for(int i=0; i<arr.length; i++){
            for(int j=i+1; j<arr.length; j++){
                if(arr[i]>arr[j]){
                    ans++;
                }
            }
        }
        return ans;
    }

    public static int[] generateRandomArray(int maxSize, int maxValue) {
        int[] arr = new int[(int) ((maxSize + 1) * Math.random())];
        for (int i = 0; i < arr.length; i++) {
            arr[i] = (int) ((maxValue + 1) * Math.random()) - (int) (maxValue * Math.random());
        }
        return arr;
    }

    // for test
    public static int[] copyArray(int[] arr) {
        if (arr == null) {
            return null;
        }
        int[] res = new int[arr.length];
        for (int i = 0; i < arr.length; i++) {
            res[i] = arr[i];
        }
        return res;
    }

    // for test
    public static boolean isEqual(int[] arr1, int[] arr2) {
        if ((arr1 == null && arr2 != null) || (arr1 != null && arr2 == null)) {
            return false;
        }
        if (arr1 == null && arr2 == null) {
            return true;
        }
        if (arr1.length != arr2.length) {
            return false;
        }
        for (int i = 0; i < arr1.length; i++) {
            if (arr1[i] != arr2[i]) {
                return false;
            }
        }
        return true;
    }

    // for test
    public static void printArray(int[] arr) {
        if (arr == null) {
            return;
        }
        for (int i = 0; i < arr.length; i++) {
            System.out.print(arr[i] + " ");
        }
        System.out.println();
    }

    // for test
    public static void main(String[] args) {
        int testTime = 500000;
        int maxSize = 100;
        int maxValue = 100;
        System.out.println("测试开始");
        for (int i = 0; i < testTime; i++) {
            int[] arr1 = generateRandomArray(maxSize, maxValue);
            int[] arr2 = copyArray(arr1);
            if (reverPairNumber(arr1) != comparator(arr2)) {
                System.out.println("Oops!");
                printArray(arr1);
                printArray(arr2);
                break;
            }
        }
        System.out.println("测试结束");
    }
}