排序算法之归并排序

<pre name="code" class="cpp">// MergeSort.cpp : Defines the entry point for the console application.//#include "stdafx.h"// SelectSort.cpp : Defines the entry point for the console application.//#include "iostream"using namespace std;// 归并排序：假设序列含义n 个记录，则可以看成是n 个长度均为1 的有序子序列(长度为1，当然有序), 然后两两归并，得到n/2 个长度为2 和 (n/2 - [n/2])个长度为1 的子序列（[x]表示向上取整），// 继续两两归并，直到得到长度为n 的有序序列为止;// 将x[s..m], s[m+1,...t] 归并到 r[s...t]void Merge(double x[], double r[], int s, int m, int t){// 将r[s..m] 和 r[m+1, ...,t] 归并为 a[s..t];int i = 0;int j = 0;int k = s;for (i=s, j=m+1; i<=m && j<=t; NULL){if (x[i] < x[j]){r[k++] = x[i++];}else{r[k++] = x[j++];}}// 将剩余的拷贝， 下面两个for 循环，最终只执行一次;for (NULL; i<=m; NULL){r[k++] = x[i++];}for (NULL; j<=t; NULL){r[k++] = x[j++];}}void MergeSortSub(double a[], double r[], int s, int t, int n){if (s == t){r[s] = a[s];}else{double *p = new double[n];int m = (s + t) / 2;// a[s...m] 归并到有序子序列p[s...m];MergeSortSub(a, p, s, m, n);// a[m+1...t] 归并到有序子序列p[m+1,...t];MergeSortSub(a, p, m + 1, t, n);// 将p[s..m], s[m+1,...t] 归并到 r[s...t]Merge(p, r, s, m, t);delete[] p;p = NULL;}}void MergeSort(double a[], int n){MergeSortSub(a, a, 0, n- 1, n);}int _tmain(int argc, _TCHAR* argv[]){double a[] = {2.0, 5.4, 4.0, 8.0, 3.0, 5.0, 1.0, 9.0, 7.0};int n = sizeof(a) / sizeof(*a);cout<<"排序前：\n";for each (double x in a){cout<<x<<" ";}cout<<endl;MergeSort(a, n);cout<<"排序后：\n";for each (double x in a){cout<<x<<" ";}cout<<endl;return 0;}