# Sắp xếp Trộn – Merge Sort

// 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){

return;//returns recursively

}

int m =l+ (r-l)/2;

mergeSort(arr,l,m);

mergeSort(arr,m+1,r);

merge(arr,l,m,r);

}