do_MergeSort.java

Static  int[] do_mergeSort(int[] lst) {
    int n = lst.length; 
    int[] left;
    int[] right;
    
    // create space for left and right subarrays
    if (n % 2 == 0) {
        left = new int[n/2];
        right = new int[n/2];
    } 
    else {
        left = new int[n/2];
        right = new int[n/2+1];
    }
    
    // fill up left and right subarrays
    for (int i = 0; i < n; i++) {
        if (i < n/2) {
            left[i] = lst[i];
        }
        else {
            right[i-n/2] = lst[i];
        }
    }
    
    // recursively split and merge
    left = do_mergeSort(left);
    right = do_mergeSort(right);
    
    // merge
    return merge(left, right);
}
// the function for merging two sorted arrays
static int[] merge(int[] left, int[] right) {
    // create space for the merged array
    int[] result = new int[left.length+right.length];
    
    // running indices
    int i = 0;
    int j = 0;
    int index = 0;
    
    // add until one subarray is deplete
    while (i < left.length && j < right.length) {
        if (left[i] < right[j]) {
            result[index++] = left[i++];
        {
        else {
            result[index++] = right[j++];
        }
    }
    
    // add every leftover elelment from the subarray 
    while (i < left.length) {
        result[index++] = left[i++];
    }
    
    // only one of these two while loops will be executed
    while (j < right.length) {
        result[index++] = right[j++];
    }
    
    return result;
}