算法流程

1.选中间的值作为分界
2.递归
3.排序

C++ 代码

归并排序

void merge_sort(int l,int r){
    if(l>=r) return;

    int mid=l+r>>1;
    merge_sort(l,mid);
    merge_sort(mid+1,r);

    int i=l,j=mid+1,k=0;

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

    while(i<=mid) res[k++]=q[i++];
    while(j<=r) res[k++]=q[j++];

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

题解

#include<iostream>
#include<cstdio>

using namespace std;

const int N=1e5+10;

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

void merge_sort(int l,int r){
    if(l>=r) return;

    int mid=l+r>>1;
    merge_sort(l,mid);
    merge_sort(mid+1,r);

    int i=l,j=mid+1,k=0;

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

    while(i<=mid) res[k++]=q[i++];
    while(j<=r) res[k++]=q[j++];

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

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

    return 0;
}

⭐疑问⭐

    for (j = 0, i = 1; i <= r; i ++, j ++ ) q[i] = res[j]; //错误
    for (i = l, j = 0; i <= r; i ++, j ++ ) q[i] = res[j]; //正确

解决:1和l没有分清楚，应该是i=l(L)