implement merge sort version of premaseem

Author: premaseem

src/me/premaseem/algorithm/mergesort/App.java

@@ -1,21 +1,73 @@
 package me.premaseem.algorithm.mergesort;
 
-import org.junit.Assert;
+/*
+Implement merge sort
+*/
+
+import me.premaseem.MyUtils;
 import org.junit.Test;
 
 public class MergeSort {
 
     // Test Driven Development by Aseem Jain
     @Test
     public void test() {
+        int a2[] = {10, 30, 190, 120, 110, 50, 7};
+
+        mergeSort(a2, 0, a2.length - 1);
+        MyUtils.isArrSorted(a2);
+    }
 
-        int a1[] = {40, 20, 50, 100, 40, 80};
-        int a2[] = {10, 30, 90, 70, 40 };
 
+    void mergeSort(int[] a, int l, int r) {
+        if (r > l) {
+            int m = (l + r) / 2;
+            mergeSort(a,l,m);
+            mergeSort(a,m+1,r);
+            realMerge(a, l, m, r);
+        }
     }
 
+    void realMerge(int[] a, int l, int m, int r) {
+
+        // find out array size
+        int las = m - l + 1; // add one because of zero index
+        int ras = r - m;
+        int[] la = new int[las];
+        int[] ra = new int[ras];
+
+        // populate elements
 
-    public void sort(int[] a){
+        for (int i = 0; i < las; i++) {
+            la[i] = a[l + i];
+        }
 
+        for (int i = 0; i < ras; i++) {
+            ra[i] = a[m + 1 + i]; // since m is covered in last loop, add m+1 here
+        }
+
+        // merge and put it on real array
+        int i1 = 0, i2 = 0;
+
+        for (int i = l; i <= r; i++) {
+            int lav = Integer.MAX_VALUE;
+            int rav = Integer.MAX_VALUE;
+
+            if (i1 < las) {
+                lav = la[i1];
+            }
+            if (i2 < ras) {
+                rav = ra[i2];
+            }
+
+            if (lav < rav) {
+                a[i] = lav;
+                i1++;
+            } else {
+                a[i] = rav;
+                i2++;
+            }
+        }
     }
+
 }