AcWing 787. 归并排序

# 归并排序 - 快排是不稳定的 / 而归并排序是稳定的

#include <iostream>

using namespace std;

const int N = 1e6 + 10;

int n;
int q[N], tmp[N];

void merge_sort(int q[], int l, int r){
    if(l >= r) return;
    int mid = (l + r) / 2;
    merge_sort(q, l, mid), merge_sort(q, mid + 1, r);
int k = 0, i = l, j = mid + 1;

while(i <= mid && j <= r)
    if(q[i] <= q[j]) tmp[k++] = q[i++];
        else tmp[k++] = q[j++];
    while(i <= mid) tmp[k++] = q[i++];
    while(j <= r) tmp[k++] = q[j++];

    for(i = l, j = 0; i <= r; i++, j++) q[i] = tmp[j];
}

int main(){
    scanf("%d", &n);

    for(int i = 0; i < n; i++) scanf("%d", &q[i]);

    merge_sort(q, 0, n - 1);

    for(int i = 0; i < n; i++) printf("%d ", q[i]);
    
    return 0;
}

//考试可能会问 快排是不稳定的
//归并排序是稳定的

#include <iostream>

using namespace std;

const int N = 1e6 + 10;

int n;
int q[N], tmp[N];

void merge_sort(int q[], int l, int r){
    if(l >= r) return;
    
    int mid = (l + r) / 2;
    merge_sort(q, l, mid), merge_sort(q, mid + 1, r);
    
    int k = 0, i = l, j = mid + 1;
    // k 表示tmp里面已经有多少个数了，即两个序列合并完已经合并了几个数
    
    
    // i 和 j 是两个指针
    // i 代表左半边起点 j代表右半边起点
    
    while(i <= mid && j <= r)
        if(q[i] <= q[j]) tmp[k++] = q[i++];
        else tmp[k++] = q[j++];
    while(i <= mid) tmp[k++] = q[i++];
    while(j <= r) tmp[k++] = q[j++];
    
    for(i = l, j = 0; i <= r; i++, j++) q[i] = tmp[j];
    //将 tmp 复制回 q 里面去
}


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