add 912

Author: fishercoder1534

Diff for: src/main/java/com/fishercoder/solutions/firstthousand/_912.java

@@ -1,12 +1,59 @@
 package com.fishercoder.solutions.firstthousand;
 
-import java.util.Arrays;
-
 public class _912 {
     public static class Solution1 {
+        /**
+         * Implementation of MergeSort which is a stable sort, unlike QuickSort which isn't stable.
+         */
         public int[] sortArray(int[] nums) {
-            Arrays.sort(nums);
+            //use a helder function to take in two additional parameters for the ease of recursion
+            return sort(nums, 0, nums.length - 1);
+        }
+
+        //this is the recursive function
+        private int[] sort(int[] nums, int left, int right) {
+            //this condition keeps dividing the array until nums becomes one individual item and then it goes back to the call stack
+            if (left < right) {
+                int mid = left + (right - left) / 2;
+                sort(nums, left, mid);
+                sort(nums, mid + 1, right);
+                merge(nums, left, mid, right);
+            }
             return nums;
         }
+
+        private void merge(int[] nums, int left, int mid, int right) {
+            int leftSize = mid - left + 1;
+            int rightSize = right - mid;
+            //use two temp array to copy the original values in the input before we overwrite them
+            int[] leftHalf = new int[leftSize];
+            int[] rightHalf = new int[rightSize];
+            for (int i = 0; i < leftSize; i++) {
+                //this index is key: it should be nums[left + i] as it should start from left instead of zero
+                leftHalf[i] = nums[left + i];
+            }
+            for (int i = 0; i < rightSize; i++) {
+                //similarly, this index is key as well: it should be nums[mid + i + 1] instead of starting from zero
+                rightHalf[i] = nums[mid + i + 1];
+            }
+            int i = 0;
+            int j = 0;
+            //again, this index k = left is key, it should start from left instead of 0
+            int k = left;
+            while (i < leftSize && j < rightSize) {
+                if (leftHalf[i] < rightHalf[j]) {
+                    nums[k++] = leftHalf[i++];
+                } else {
+                    nums[k++] = rightHalf[j++];
+                }
+            }
+
+            while (i < leftSize) {
+                nums[k++] = leftHalf[i++];
+            }
+            while (j < rightSize) {
+                nums[k++] = rightHalf[j++];
+            }
+        }
     }
 }