面试题 36: 数组中的逆序对

一. 题目

在数组中的两个数字如果前面一个数字大于后面的数字,则这两个数字组成一个逆序对.输入一个数组,求出这个数组中的逆序对的总数.

代码请到我的代码库中下载 Point2Offer

二. 代码

package ween_2;

/**难度系数:****
 * 剑指offer: 数组中的逆序对
 * 方法:归并排序思想
 * 测试用例:功能测试(123456,135246,654321,52135,31,1)
 * 特殊输入测试(数组为空)
 * @author dingding
 * Date:2017-6-22 7:30
 * Declaration: All Rights Reserved!
 */
public class No36 {

    public static void main(String[] args) {
        test1();
        test2();
        test3();
        test4();
        test5();
        test6();
        test7();
    }

    //逆序对
    private static int inversePairs(int[] arr,int length){
        if (arr == null || arr.length<1) {
            return 0;
        }            

        int[] copy = new int[length];    //辅助数组
        for (int i=0;i<length;i++){
            copy[i]=arr[i];
        }

        int count = inversePairsCore(arr,copy,0,length-1);   //计算逆序对
        return count;
    }

    private static int inversePairsCore(int[] arr, int[] copy, int start, int end) {
        //递归结束条件
        if (start == end) {
            //copy[start] = arr[start];  //分解成单元素后的处理
            return 0;
        }
        int length = (end - start)/2;
        int left = inversePairsCore(copy, arr, start, start+length);  //会一直执行到完成 (排序,注意参数1和2是反的)
        int right = inversePairsCore(copy, arr,start+length+1, end);

        /*合并,排序,并统计逆序对*/
        //i初始化为前半段最后一个数字的坐标
        int i = start+length;
        //j初始化为后半段最后一个数字的坐标
        int j = end;
        int indexCopy = end;  //辅助数组的指针位置
        int count = 0;
        //可把i,j看出是两个指针
        while (i >= start && j>=start+length+1){
            if (arr[i]>arr[j]) {
                copy[indexCopy--]=arr[i--];
                count += j-start-length;
            }else {
                copy[indexCopy--]=arr[j--];
            }
        }

        //右半边用尽,取左半边元素
        for (;i>=start;i--){
            copy[indexCopy--]=arr[i];
        }
        //左半边用尽,取右半边元素
        for (;j>=start+length+1;j--){
            copy[indexCopy--]=arr[j];
        }

        return left+right+count;
    }

    /*==================测试用例==============*/
    private static void test(int[] data,int length,int expected){
        if(inversePairs(data, length)== expected)
            System.out.println("Passed.");
        else
            System.out.println("Failed.");
    }
    private static void test1() {
        int[] data = {7,5,6,4};
        int expected = 5;
        test(data, data.length, expected);
    }

    private static void test2() {
        int[] data = {6, 5, 4, 3, 2, 1};
        int expected = 15;
        test(data, data.length, expected);
    }

    private static void test3() {
        int[] data = {1, 2, 3, 4, 5, 6};
        int expected = 0;
        test(data, data.length, expected);
    }

    private static void test4() {
        int[] data = {1};
        int expected = 0;   
        test(data, data.length, expected);
    }

    private static void test5() {
        int[] data = {1, 2};
        int expected = 0;   
        test(data, data.length, expected);
    }

    private static void test6() {
        int[] data = {2, 1};
        int expected = 1;   
        test(data, data.length, expected);
    }

    private static void test7() {
        int[] data = {1, 2, 1, 2, 1};
        int expected = 3;
        test(data, data.length, expected);
    }

}

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有不妥当之处,麻烦告知:D