101_merge_sort.cpp

// https://practice.geeksforgeeks.org/problems/merge-sort/1

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

void merge(int arr[], int l, int m, int r);

/* l is for left index and r is right index of the
   sub-array of arr to be sorted */
void mergeSort(int arr[], int l, int r)


/* Function to print an array */
void printArray(int arr[], int size)
{
	int i;
	for (i = 0; i < size; i++)
		printf("%d ", arr[i]);
	printf("\n");
}

int main()
{
#ifndef ONLINE_JUDGE
	freopen("input.txt", "r", stdin);
	freopen("output.txt", "w", stdout);
#endif
	int n, T, i;

	scanf("%d", &T);

	while (T--) {

		scanf("%d", &n);
		int arr[n + 1];
		for (i = 0; i < n; i++)
			scanf("%d", &arr[i]);

		mergeSort(arr, 0, n - 1);
		printArray(arr, n);
	}
	return 0;
}


/* The task is to complete merge() which is used
   in below mergeSort() */
/*   l is for left index and r is right index of the
   sub-array of arr to be sorted
void mergeSort(int arr[], int l, int r) {
    if (l < r)   {
        int m = l+(r-l)/2;
        mergeSort(arr, l, m);
        mergeSort(arr, m+1, r);
        merge(arr, l, m, r);
    }
} */

// Merges two subarrays of arr[].  First subarray is arr[l..m]
// Second subarray is arr[m+1..r]
void merge(int arr[], int l, int m, int r)
{
	int n1 = m - l + 1, n2 = r - m;
	int L[n1], R[n2];

	for (int i = 0; i < n1; ++i)
		L[i] = arr[i + l];

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

	int i = 0, j = 0, k = l;
	while (i < n1 && j < n2)
	{
		if (L[i] < R[j])
		{
			arr[k] = L[i];
			i++;
		}
		else
		{
			arr[k] = R[j];
			j++;
		}
		k++;
	}

	while (i < n1)
	{
		arr[k] = L[i];
		i++;
		k++;
	}

	while (j < n2)
	{
		arr[k] = R[j];
		j++;
		k++;
	}
}

void mergeSort(int arr[], int l, int r)
{
	if (l < r)
	{
		// Same as (l+r)/2, but avoids overflow for
		// large l and h
		int m = l + (r - l) / 2;

		// Sort first and second halves
		mergeSort(arr, l, m);
		mergeSort(arr, m + 1, r);

		merge(arr, l, m, r);
	}
}