Merge sort【1月18日学习笔记】

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//Merge sort
#include<iostream>
using namespace std;

void merge(int L[], int R[], int A[], int nL, int nR) {//将两个已排序数组合并填入
	int i = 0, j = 0, k = 0;//i，j为未拾取元素索引，k为归并数组索引
	while (i < nL && j < nR) {
		if (L[i] < R[j]) {
			A[k] = L[i];
			i++;
		}
		else {
			A[k] = R[j];
			j++;
		}
		k++;
	}//在两个数组中选取未拾取元素最小者填入归并数组
	while (i < nL) {
		A[k] = L[i];
		i++;
		k++;
	}
	while (j < nR) {
		A[k] = R[j];
		j++;
		k++;
	}//有剩余元素数组全部填入
}

void mergesort(int A[], int n) {
	if (n < 2) return;//中止条件
	int mid = n / 2;//从中间分为两组
	int* L = new int[mid];
	int* R = new int[n - mid];//动态分配

	for (int i = 0; i < mid; i++) L[i] = A[i];//左边填入
	for (int i = mid; i < n; i++) R[i - mid] = A[i];//右边填入

	mergesort(L, mid);  // sorting the left subarray
	mergesort(R, n - mid);  // sorting the right subarray
	merge(L, R, A, mid, n - mid);//归并填入
	delete []L;
	delete []R;//不要忘记删除动态内存
}//时间复杂度：O(nlogn)
 //空间复杂度：O(n)

int main() {
	int A[] = { 2,7,4,1,5,3 };
	mergesort(A, 6);
	for (int i = 0; i < 6; i++)  cout << A[i] << " ";
	cout << endl;
}