归并排序

       分治法的另一种经典引用，这里贴上递归和非递归的代码。（递归版本是为了防止我忘记太多的C++知识，所以应用了模板类）

 

递归版本：

#include<iostream>
#define T double

using namespace std;

T a[10000] = {0};
int N;

template <typename Type>
void Merge(int left, int middle, int right){
	int len = right - left + 1;
	int i = left, j = middle + 1, m = 0;
	T *b = new T[len];
	for(m = 0; i <= middle && j <= right; m++){
		if(a[i] < a[j]) b[m] = a[i++];
		else b[m] = a[j++];
	}
	for(; i <= middle; m++, i++) b[m] = a[i];
	for(; j <= right; m++, j++) b[m] = a[j];
	for(int i = 0; i < len; i++){
		a[left + i] = b[i];
	}
}

template <typename Type>
void MergeSort(int left, int right){
	if(left < right){
		int middle = (left + right) / 2;
		MergeSort<Type>(left, middle);
		MergeSort<Type>(middle + 1, right);
		Merge<Type>(left, middle, right);
	}
}

int main(){
	cin >> N;
	for(int i = 0; i < N; i++) cin >> a[i];
	MergeSort<T>(0, N - 1);
	for(int i = 0; i < N; i++){
		if(i == 0) cout << a[i];
		else cout << ' ' << a[i];
	}
}

 

 

非递归版本：

#include<iostream>

#define MAX 0x7FFFFFFF

using namespace std;

void MergeSort(int n);
void MergePass(int step, int n);
void Merge(int left, int middle, int right);

int a[10000] = {0};
int N;

void MergeSort(int n){
	//分组，每组进行一次归并排序
	for(int i = 1; i < n; i *= 2)
		MergePass(i, n);
}

void MergePass(int step, int n){
	//划分, 合并成2 * step的有序数组
	int i, j;
	for(i = 0; i + 2 * step - 1 < n;i = j + 1){
		j = i + 2 * step - 1;
		Merge(i, i + step - 1, i + 2 * step - 1);
	}
	if(n - i > step){
		//剩下元素大于step小于2 * step
		Merge(i, i + step - 1, n - 1);
	}else if(n - i > 0){
		//剩余元素小于step个
		Merge(i, i, n - 1);
	}
}

void Merge(int left, int middle, int right){
	if(left == right) return ;
	int len = right - left + 1;
	int *b = new int[len];
	for(int i = 0, m = left, n = middle + 1; i < len; i++){
		if(a[m] < a[n]){
			b[i] = a[m];
			if(m < middle) m++;
			else a[m] = MAX;
		}else{
			b[i] = a[n];
			if(n < right) n++;
			else a[n] = MAX;
		}
	}
	for(int i = 0; i < len; i++){
		a[left + i] = b[i];
	}	
}

int main(){
	cin >> N;
	for(int i = 0; i < N; i++) cin >> a[i];
	MergeSort(N);
	for(int i = 0; i < N; i++){
		if(i == 0) cout << a[i];
		else cout << ' ' << a[i];
	}
}