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归并排序(Merge sort)用到了分治思想，即分-治-合三步，算法平均时间复杂度是o（nlgn）.
（一）算法实现
1 private void merge_sort(int[] array, int first, int last) { 2 if (first + 1 < last) { 3 int mid = (first + last) / 2; 4 merge_sort(array, first, mid); 5 merge_sort(array, mid, last); 6 7 // merge 8 int i = first; 9 int j = mid; 10 int[] tempArray = new int[last - first]; 11 for (int k = 0; k < tempArray.length; k++) { 12 if (j == last) { 13 tempArray[k] = array[i++]; 14 } else if (i == mid) { 15 tempArray[k] = array[j++]; 16 } else if (array[i] < array[j]) { 17 tempArray[k] = array[i++]; 18 } else if (array[i] >= array[j]) { 19 tempArray[k] = array[j++]; 20 } else { 21 throw new RuntimeException("Merge sort error."); 22 } 23 } 24 System.arraycopy(tempArray, 0, array, first, last - first); 25 } 26 }
1）算法不是原地排序，合并时需要额外空间
2）算法平均时间复杂度是o（nlgn）
（二）仿真结果
**************************************************
Number to Sort is:2500
Array to sort is:{633649,71377,977368,56277,635475,755791,746668,373470,820128,669757...}
Cost time of 【MergSort】 is(milliseconds):1
Sort result of 【MergSort】:{695,711,935,1074,1571,1681,2434,3967,4104,4452...}
**************************************************
Number to Sort is:25000
Array to sort is:{62806,677129,876202,279782,662046,978299,979879,374196,303999,484652...}
Cost time of 【MergSort】 is(milliseconds):5
Sort result of 【MergSort】:{15,34,74,94,131,141,316,372,375,417...}
**************************************************
Number to Sort is:250000
Array to sort is:{617507,750026,202102,267444,690991,638097,868240,413838,774218,463780...}
Cost time of 【MergSort】 is(milliseconds):67
Sort result of 【MergSort】:{6,9,19,28,31,39,44,45,56,59...}
相关代码：
1 package com.cnblogs.riyueshiwang.sort; 2 3 import java.util.Arrays; 4 5 public class MergeSort extends abstractSort { 6 7 @Override 8 protected void sort(int[] toSort) { 9 merge_sort(toSort, 0, toSort.length); 10 } 11 12 private void merge_sort(int[] array, int first, int last) { 13 if (first + 1 < last) { 14 int mid = (first + last) / 2; 15 merge_sort(array, first, mid); 16 merge_sort(array, mid, last); 17 18 // merge 19 int i = first; 20 int j = mid; 21 int[] tempArray = new int[last - first]; 22 for (int k = 0; k < tempArray.length; k++) { 23 if (j == last) { 24 tempArray[k] = array[i++]; 25 } else if (i == mid) { 26 tempArray[k] = array[j++]; 27 } else if (array[i] < array[j]) { 28 tempArray[k] = array[i++]; 29 } else if (array[i] >= array[j]) { 30 tempArray[k] = array[j++]; 31 } else { 32 throw new RuntimeException("Merge sort error."); 33 } 34 } 35 System.arraycopy(tempArray, 0, array, first, last - first); 36 } 37 } 38 39 public static void main(String[] args) { 40 for (int j = 0, n = 2500; j < 3; j++, n = n * 10) { 41 System.out 42 .println("**************************************************"); 43 System.out.println("Number to Sort is:" + n); 44 int[] array = CommonUtils.getRandomIntArray(n, 1000000); 45 System.out.print("Array to sort is:"); 46 CommonUtils.printIntArray(array); 47 48 int[] array1 = Arrays.copyOf(array, n); 49 new MergeSort().sortAndprint(array1); 50 } 51 } 52 }
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原文地址：http://www.cnblogs.com/riyueshiwang/p/4591186.html
