MergeSort.java

public class MergeSort {
    public int[] sortArray(int[] nums){
        mergeSort(nums, 0, nums.length - 1);
        return nums;
    }
    private void mergeSort(int[] nums, int left, int right){
        if(left < right){
            int mid = left + (right - left) / 2;
            mergeSort(nums, left, mid);
            mergeSort(nums, mid + 1, right);
            merge(nums, left, mid, right);
        }
    }
    private void merge(int[] nums, int left, int mid, int right){
        int n1 = mid - left + 1;
        int n2 = right - mid;
        int[] L = new int[n1];
        int[] R = new int[n2];
        for(int i = 0; i < n1; i++){
            L[i] = nums[left + i];
        }
        for(int i = 0; i < n2; i++){
            R[i] = nums[mid + 1 + i];
        }
        int i = 0, j = 0, k = left;
        while(i < n1 && j < n2){
            if(L[i] <= R[j]){
                nums[k] = L[i];
                i++;
            }else{
                nums[k] = R[j];
                j++;
            }
            k++;
        }
        while(i < n1){
            nums[k] = L[i];
            i++;
            k++;
        }
        while(j < n2){
            nums[k] = R[j];
            j++;
            k++;
        }
    }
}