25、JAVA歸併排序

歸併排序時間複雜度：最好最壞平均都是nlogn空間複雜度：n穩定性：穩定的 歸併排序的用途：1、排序

　　（速度僅次於快速排序，但較穩定）

2、求逆序對數【這個單獨出個筆試題，求逆序對數】

　　具體思路是，在歸併的過程中計算每個小區間的逆序對數，進而計算出大區間的逆序對數（也可以用樹狀數組來求解）

 

JAVA實現歸併排序：

 

import java.util.Arrays;public class MergeSort {public static void main(String args[]){int a[] = {4,4,1,6,3};mergeSort(a,0,a.length-1);/*for(int i=0;i<a.length-1;i++)System.out.println(a[i]);*/  System.out.println(Arrays.toString(a));}private static void mergeSort(int a[],int start,int end){//int mid = (start+end)/2;//int[] temp = new int[a.length];int mid=(start&end)+((start^end)>>1);if(start<end){//跳出遞迴，原來忘記寫這一句，導致棧溢出mergeSort(a,start,mid);mergeSort(a,mid+1,end);merge(a,start,mid,mid+1,end);}}private static void merge(int a[],int start1,int end1,int start2,int end2){int i = start1;int j = start2;//int k = 0;//k與此處保持一致，System.arraycopy(temp, 0, a, start1, end2-start1+1);int k = start1;//k與此處保持一致，System.arraycopy(temp, start1, a, start1, end2-start1+1);int temp[] = new int[a.length+1];while(i<=end1 && j<=end2){if(a[i] < a[j]){temp[k++] = a[i++];//temp[k] = a[i];//i++;}else{temp[k++] = a[j++];//temp[k] = a[j];//j++;}//k++;}while(i<=end1){temp[k++] = a[i++];}while(j<=end2){temp[k++] = a[j++];}//k = start1;//for(int element:temp){//把臨時數組元素賦值給原數組//a[k++] = element;System.arraycopy(temp, start1, a, start1, end2-start1+1);}}

 

 

刪除注釋後的完整可運行代碼：

import java.util.Arrays;public class MergeSort {public static void main(String args[]){int a[] = {4,4,1,6,3};mergeSort(a,0,a.length-1);/*for(int i=0;i<a.length-1;i++)System.out.println(a[i]);*/  System.out.println(Arrays.toString(a));}private static void mergeSort(int a[],int start,int end){//int mid = (start+end)/2;int mid=(start&end)+((start^end)>>1);if(start<end){//跳出遞迴，原來忘記寫這一句，導致棧溢出mergeSort(a,start,mid);mergeSort(a,mid+1,end);merge(a,start,mid,mid+1,end);}}private static void merge(int a[],int start1,int end1,int start2,int end2){int i = start1;int j = start2;int k = start1;//k與此處保持一致，System.arraycopy(temp, start1, a, start1, end2-start1+1);int temp[] = new int[a.length+1];while(i<=end1 && j<=end2){if(a[i] < a[j]){temp[k++] = a[i++];}else{temp[k++] = a[j++];}}while(i<=end1){temp[k++] = a[i++];}while(j<=end2){temp[k++] = a[j++];}System.arraycopy(temp, start1, a, start1, end2-start1+1);//從temp數組複製到a數組}}