[Java] Merge sort

merge_sort.java

// in-line recursive merge sort
public static void mergeSort(int[] nums) {
    subMergeSort(nums, 0, nums.length-1, new int[nums.length]);
}
private static void subMergeSort(int[] nums, int start, int end, int[] swap) {
    if (start == end) {
        return;
    }
    // sort sub-lists
    int pivot = (start + end) / 2;

    // divine and conquer so the left and right lists are sorted
    if (end - start > 1) {
        subMergeSort(nums, start, pivot, swap);
        subMergeSort(nums, pivot + 1, end, swap);
    }

    // merge left and right sorted sub lists
    // results are stored in swap to avoid overwriting
    int left = start;
    int right = pivot+1;
    for (int i = 0; i < end-start+1; i++) {
        if (left > pivot) {
            swap[i] = nums[right++];
        } else if (right > end) {
            swap[i] = nums[left++];
        } else {
            // both are in bounds, take the smallest
            if (nums[left] < nums[right]) {
                swap[i] = nums[left++];
            } else {
                swap[i] = nums[right++];
            }
        }
    }

    // swap back
    for (int i = 0; i < end - start + 1; i++) {
        nums[start+i] = swap[i];
    }
}