merge sort algo.cpp

// C++ program for Merge Sort worst case :O(n log(n))for all cases 
#include<iostream> 
using namespace std; 

// 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; 
	int n2 = r - m; 

	// Create temp arrays 
	int L[n1], R[n2]; 

	// Copy data to temp arrays L[] and R[] 
	for(int i = 0; i < n1; i++) 
		L[i] = arr[l + i]; 
	for(int j = 0; j < n2; j++) 
		R[j] = arr[m + 1 + j]; 

	// Merge the temp arrays back into arr[l..r] 
	
	// Initial index of first subarray 
	int i = 0; 
	
	// Initial index of second subarray 
	int j = 0; 
	
	// Initial index of merged subarray 
	int k = l; 
	
	while (i < n1 && j < n2) 
	{ 
		if (L[i] <= R[j]) 
		{ 
			arr[k] = L[i]; 
			i++; 
		} 
		else
		{ 
			arr[k] = R[j]; 
			j++; 
		} 
		k++; 
	} 

	// Copy the remaining elements of 
	// L[], if there are any 
	while (i < n1) 
	{ 
		arr[k] = L[i]; 
		i++; 
		k++; 
	} 

	// Copy the remaining elements of 
	// R[], if there are any 
	while (j < n2) 
	{ 
		arr[k] = R[j]; 
		j++; 
		k++; 
	} 
} 

// 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) 
	{ 
		
		// 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); 
	} 
} 

// UTILITY FUNCTIONS 
// Function to print an array 
void printArray(int A[], int size) 
{ 
	for(int i = 0; i < size; i++) 
		cout << A[i] << " "; 
} 

// Driver code 
int main() 
{ 
	int arr[] = { 12, 11, 13, 5, 6, 7 }; 
	int arr_size = sizeof(arr) / sizeof(arr[0]); 

	cout << "Given array is \n"; 
	printArray(arr, arr_size); 

	mergeSort(arr, 0, arr_size - 1); 

	cout << "\nSorted array is \n"; 
	printArray(arr, arr_size); 
	return 0; 
}