MergeSort

#include<iostream>

using namespace std;

void __merge(int arr[],int l,int mid,int r){
    //auxiliary array
    int *aux=new int[r-l+1];

    for(int i=l;i<=r;i++) {
        //copy datas
        aux[i - l] = arr[i];

    }

    int i=l;
    //int mid=l+(r-l)/2;
    int j=mid+1;
    for(int k=l;k<=r;k++){
        //left branch out
        if(i>mid){
            arr[k]=aux[j-l];j++;
        }

        else if(j>r){
            arr[k]=aux[i-l];i++;
        }

        else if(aux[i-l]<aux[j-l]){
            arr[k]=aux[i-l];i++;
        }

        else {
            arr[k]=aux[j-l];j++;
        }
    }

    delete[] aux;
}
//arr[l...r]
void __mergeSort(int arr[],int l,int r){
    //recurrence end condition
    if(l>=r){
        return;
    }
    int mid=(l+r)/2;
    //divide
    __mergeSort(arr,l,mid);
    __mergeSort(arr,mid+1,r);
    //merge(conquer)
    __merge(arr,l,mid,r);

}

void merge(int arr[],int n){
    __mergeSort(arr,0,n-1);
}

int main(){
    int arr[]={3,2,4,1,8,7,9,6,5,0};
    merge(arr,10);
    for (int i = 0; i < 10; i++)
    {
        cout<<arr[i]<<" ";
    }

    cout<<endl;
    return 0;
}

//merge from bottom to up
#include<iostream>

using namespace std;

void __merge(int arr[], int l, int mid, int r) {
    //auxiliary array
    int *aux = new int[r - l + 1];


    for (int i = l; i <= r; i++) {
        //copy datas
        aux[i - l] = arr[i];
    }


    int i = l;
    int j = mid + 1;
    for (int k = l; k <= r; k++) {

        if (j > r) {
            arr[k] = aux[i - l];
            i++;
        }

        else if (i > mid) {
            arr[k] = aux[j - l];
            j++;
        }

        else if (aux[i - l] < aux[j - l]) {
            arr[k] = aux[i - l];
            i++;
        }

        else {
            arr[k] = aux[j - l];
            j++;
        }
    }

    delete[] aux;
}

void mergeBu(int arr[], int n) {
    //divide
    for (int sz = 1; sz < n; sz += sz) {
        //merge arr[i...i+sz-1]  arr[i+sz...i+sz+sz-1]
        //if i+sz exists,i+sz-1<n-1
        for (int i = 0; i + sz < n; i += sz + sz) {
            __merge(arr, i, i + sz - 1, min(i + sz + sz - 1, n - 1));
        }
    }
}


int main() {
    int arr[] = {3, 2, 4, 1, 8, 7, 5, 9, 0, 6};
    mergeBu(arr, 10);
    for (int i = 0; i < 10; i++) {
        cout << arr[i] << " ";
    }

    cout << endl;
    return 0;
}