合并排序 | C++ - 分段错误

Question

I thought I'd revise my concepts by coding some basic DS and Algos. After a great pace, I am kinda stuck now and not being able to identify my mistake. I am getting a Segmentation Fault in the below code result. Any help would be great!

#include<bits/stdc++.h>
using namespace std;

void merge(int *arr, int s, int m, int e){
    int n1 = m-s, n2 = e-m+1;
    int L[n1], R[n2];
    for(int i=0; i<n1; i++)
        L[i] = arr[s+i];
    for(int i=0; i<n2; i++)
        R[i] = arr[m+i];
    
    int p1 = 0, p2 = 0, k = s;
    while(p1<n1 && p2<n1){
        if(L[p1] <= R[p2])
            arr[k++] = L[p1++];
        else
            arr[k++] = R[p2++];
    }

    while(p1<n1)
        arr[k++] = L[p1++];
    while(p2<n2)
        arr[k++] = R[p2++];
    return;
}


void mergeSort(int *arr, int s, int e){
    if(s>=e) return;
    int m = (s+e)/2;
    mergeSort(arr, s, m-1);
    mergeSort(arr, m, e);
    merge(arr, s, m, e);
    return;
}
int main(){
    int n;
    cin>>n;
    int arr[n];
    for(int i=0; i<n; i++)
        cin>>arr[i];
    mergeSort(arr, 0, n-1);

    for(int i=0; i<n; i++)
        cout<<arr[i]<<" ";
    cout<<endl;

    return 0;
}

Thanks in advance!

Answer 1

The segmentation error is because you have an infinite recursion loop. There were some problems with the indices and one typo where you meant n2 but wrote n1.

#include <iostream>

using namespace std;

void merge(int *arr, int s, int m, int e){
    int n1 = m-s+1, n2 = e-m;
    int L[n1], R[n2];

    for(int i=0; i<n1; i++)
        L[i] = arr[s+i];
    for(int i=0; i<n2; i++)
        R[i] = arr[m+i+1];

    int p1 = 0, p2 = 0, k = s;
    while(p1<n1 && p2<n2){
        if(L[p1] <= R[p2])
            arr[k++] = L[p1++];
        else
            arr[k++] = R[p2++];
    }

    while(p1<n1)
        arr[k++] = L[p1++];
    while(p2<n2)
        arr[k++] = R[p2++];
    return;
}

void mergeSort(int *arr, int s, int e){
    if(s>=e) return;
    int m = (s+e)/2;
    mergeSort(arr, s, m);
    mergeSort(arr, m+1, e);
    merge(arr, s, m, e);
    return;
}

int main() {
    int n;

    cout << "Select value for N:";
    cin>>n;
    int arr[n];

    cout << "Reading input:";
    for(int i=0; i<n; i++)
        cin >> arr[i];

    cout << "Your input is:" << endl;
    for(int i=0; i<n; i++)
        cout << arr[i]<<" ";
    cout << endl;

    cout << "Running merge sort" << endl;
    mergeSort(arr, 0, n-1);

    cout << "Final array:" << endl;
    for(int i=0; i<n; i++)
        cout<<arr[i]<<" ";
    cout << endl;

    return 0;
}