Create merge sort

Author: Ruby123-creator

merge sort

@@ -0,0 +1,72 @@
+// "static void main" must be defined in a public class.
+public class Main {
+    public static void main(String[] args) {
+        int[] arr = {2, 1, 3, 1, 2};
+        count = 0;
+        int[] res = mergeSort(arr, 0, arr.length-1);
+        System.out.println(count);
+        System.out.println(Arrays.toString(res));
+    }
+    static int count;
+    static int[] mergeSort(int[] arr, int lo, int hi){
+        if(lo == hi){
+            // int[] bs = new int[1];
+            // bs[0] = arr[lo];
+            // return bs;
+            return new int[]{arr[lo]};
+        }
+        // if(lo < hi){
+            int mid = (lo + hi)/2;
+            int[] left = mergeSort(arr, lo, mid);
+            int[] right = mergeSort(arr, mid+1, hi);
+
+            int[] res = mergeTwoSortedArray(left, right);
+            return res;
+        // }else {
+            // return new int[]{arr[lo]};
+        // }
+        
+    }
+    // cnt = 1
+    // {2, 1, 3, 1, 2};
+    // 1 2 3       1 2
+    // 
+    // 
+    static int[] mergeTwoSortedArray(int[] left, int[] right){
+        int n = left.length;
+        int m = right.length;
+        
+        // 1 2 3 5
+        // 2 2 4 
+        
+        // 1 2 2 2 4 5
+        
+        
+        int[] res = new int[n+m];
+        int i = 0;
+        int j = 0;
+        int k = 0;
+        while(i<n && j<m){
+            if(left[i]<=right[j]){
+                res[k++] = left[i++];
+            }else {
+                count += n - i;
+                res[k++] = right[j++];
+            }
+        }
+        
+        while(i<n){
+            res[k++] = left[i++];
+        }
+        while(j<m){
+            res[k++] = right[j++];
+        }
+        return res;
+    }
+}
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