1 public class MergeSort {
 2 
 3     public static void sortIntegers(int[] array) {
 4         // write your code here
 5         if (array == null || array.length ==0 ) {
 6             return ;
 7         }
 8         //!!!write outside could save some space than writing inside the merge function
 9
 
         int[] helper = new int[array.length] ;
10         mergeSort(array, helper, 0, array.length-1) ;
11     }
12 
13     private static void mergeSort(int[] array, int[] helper, int left, int right) {
14         //exist recursion
15         if (left>=right) {
16             return;
17         }
18
 
         int mid = left + (right-left)/2 ;
19         /*
20         * 註意這裏不用的原因 [0,1] start = 0 mid = 0(1/2 in java = 0) end = 1
21         * mergeSort(A, helper, start, mid-1 )  (0,-1) will exit in the mergeSort so its ok
22         * mergeSort(A, helper, mid, end, ),  (0,1) will become the same as the input, input size not decrease, so timeout!
 
23         * */
24         mergeSort(array, helper, left, mid );
25         mergeSort(array, helper,mid +1, right);
26         merge(array, helper, left, mid, right);
27     }
28     //merge //這兩邊往中間夾的算法，都要考慮一邊先結
29     private static void merge(int[] array,int[] helper, int left, int mid, int right) {
30         /*note
31         1) here both the left and right are index! so use <= right
32         2) when it comes to merge, left part and right part already sorted in the array!
33         [ 1,3,5 ; 6,7,9]
34         * */
35         //copy [1,3,5 ; 6,7,9] from array to helper to save ori. value, so later we could use original val. from it to overwrite array
36         for (int i = left; i <= right; i++) {
37             helper[i] = array[i] ;
38         }
39         // use left as the pointer for array, use leftIndex and rightIndex as two pointers for helper array
40         int leftIndex = left ;
41         int rightIndex = mid + 1 ;
42         while (leftIndex <=mid && rightIndex <= right){
43             if (helper[leftIndex] <= helper[rightIndex]){
44                 array[left++] = helper[leftIndex++] ;
45             } else{
46                 array[left++] = helper[rightIndex++] ;
47             }
48         }
49         //two pointers related ques. always have to do checking for one side left situation
50         //[ 6,7,9 ; 1,3,5 ]: the array will be [1,3,5;1,3,5], use the helper which keep the original value to overwrite it
51         while(leftIndex<=mid){
52             array[left++] = helper[leftIndex++] ;
53         }
54         //[ 1,3,5 ; 6,7,9] === in this case, there would be left on the right but no need handle at all, the array already [1,3,5;6,7,9]
55 
56     }
57 
58     public static void main(String[] args){
59         int[] array = {6,7,9 , 1,3,5} ;
60         sortIntegers(array) ;
61         print(array);
62 
63     }
64     public static void print(int[] arr){
65         for (int i = 0; i < arr.length; i++) {
66             System.out.println(arr[i]);
67         }
68     }
69 }

Merge Sort