Merge Sort

#include<bits/stdc++.h>
#define endl "\n"

using namespace std;

void merge_them(vector<int> &ara, int L, int mid, int R) {
    vector<int> partial_sort;
    int L1 = L, R1 = mid, L2 = mid + 1, R2 = R;

    while(L1<=R1 && L2<=R2) {
        if(ara[L1] <= ara[L2]) {
            partial_sort.push_back(ara[L1]);
            L1++;
        } else {
            partial_sort.push_back(ara[L2]);
            L2++;
        }
    }
    while(L1<=R1) {
        partial_sort.push_back(ara[L1]);
        L1++;
    }
    while(L2<=R2) {
        partial_sort.push_back(ara[L2]);
        L2++;
    }
    for(int i = 0, j = L; i<partial_sort.size(); i++, j++) {
        ara[j] = partial_sort[i];
    }
}

void merge_sort(vector<int>&arr, int L, int R) {
    if (L >= R) return;
    int M = (L + R) / 2;

    /// arr -> [6, 2, 5, 11, 1, -1, 7, 3]

    merge_sort(arr, L, M);
    /// arr[L,...,M] is sorted. arr -> [2, 5, 6, 11, 1, -1, 7, 3]

    merge_sort(arr, M + 1, R);
    /// arr[M + 1,...,R] is sorted. arr -> [2, 5, 6, 11, -1, 1, 3, 7]

    merge_them(arr, L, M, R);
    /// arr[L,...,R] -> [-1, 1, 2, 3, 5, 6, 7, 11]
}

void merge_sort(vector<int>&arr) {
    merge_sort(arr, 0, arr.size() - 1);
}

int main()
{
    int n;
    cin >> n;
    vector<int> arr(n);
    for (int i = 0; i < n; i++)
    {
        cin >> arr[i];
    }
    merge_sort(arr);
    if (is_sorted(arr.begin(), arr.end())) {
        cout << "SUCCESS\n";
    }
    else {
        cout << "FAILURE\n";
    }
    return 0;
}