4. 归并排序和快速排序

 1.递归形式的归并排序

#include<iostream>
using namespace std;
int n;
int *a;

void merge(int l, int mid, int r, int *b){
    int i = l, j = mid + 1, k = 0;
    while (i <= mid && j <= r) {
        if (a[i] < a[j]) {
            b[k] = a[i];
            i++;
        }
        else {
            b[k] = a[j];
            j++;
        }
        k++;
    }
    if (i <= mid) {
        while (i <= mid) {
            b[k] = a[i];
            k++;
            i++;
        }
    }
    if (j <= r) {
        while (j <= r) {
            b[k] = a[j];
            k++;
            j++;
        }
    }
}

void copy(int l, int r, int *b) {
    int j = 0;
    for (int i = l; i <= r; i++)
        a[i] = b[j++];
}

void sort(int l, int r) {
    if (l == r) return;
    
    int *b = new int[r - l + 1];
    int mid = (l + r) / 2; 
    sort(l, mid);
    sort(mid + 1, r);
    merge(l, mid, r, b);
    copy(l, r, b);
    delete[] b;
}

int main() {
    scanf("%d", &n);
    a = new int[n];
    for (int i = 0; i < n; i++)
        scanf("%d", &a[i]);
    sort(0, n - 1);
    for (int i = 0; i < n; i++)
        printf("%d ", a[i]);
    delete[] a;
    return 0;
} 

2.非递归形式的归并排序

#include<iostream>
using namespace std;
int n;
int *a;

void merge(int l, int mid, int r, int *b){
    if (r > n - 1) r = n - 1;
    if (mid > n - 1) mid = n - 1;
    int i = l, j = mid + 1, k = 0;
    while (i <= mid && j <= r) {
        if (a[i] < a[j]) {
            b[k] = a[i];
            i++;
        }
        else {
            b[k] = a[j];
            j++;
        }
        k++;
    }
    if (i <= mid) {
        while (i <= mid) {
            b[k] = a[i];
            k++;
            i++;
        }
    }
    if (j <= r) {
        while (j <= r) {
            b[k] = a[j];
            k++;
            j++;
        }
    }
}

void copy(int l, int r, int *b) {
    if (r > n - 1) r = n - 1;
    int j = 0;
    for (int i = l; i <= r; i++)
        a[i] = b[j++];
}

void sort(){
    for (int i = 1; i < n; i += i){
        for (int j = 0; j < n; j += 2*i){
            int *b = new int[i];
            merge(j, j+i-1, j+2*i-1, b);
            copy(j, j+2*i-1, b);
            delete[] b;
        }    
    }
}

int main() {
    scanf("%d", &n);
    a = new int[n];
    for (int i = 0; i < n; i++)
        scanf("%d", &a[i]);
    sort();
    for (int i = 0; i < n; i++)
        printf("%d ", a[i]);
    delete[] a;
    return 0;
}

3.快速排序

#include<iostream>
using namespace std;
int n;
int *a;

int partition(int left, int right)
{
    int i = left + 1 ;
    int j = right;
    int temp = a[left];

    while(i <= j)
    {
        while (a[i] < temp) i++;
        while (a[j] > temp ) j--;
        if (i < j) swap(a[i++], a[j--]);
        else i++;
    }
    swap(a[j], a[left]);
    return j;

}

void quick_sort(int left, int right) 
{
    if (left > right)
        return;
    int j = partition(left, right);
    quick_sort(left, j - 1);
    quick_sort(j + 1, right);
}

int main() {
    scanf("%d", &n);
    a = new int[n];
    for (int i = 0; i < n; i++)
        scanf("%d", &a[i]);
    quick_sort(0, n - 1);
    for (int i = 0; i < n; i++)
        printf("%d ", a[i]);
    delete[] a;
    return 0;
}