Added merge sort

Author: FreeTymeKiyan

Other/MergeSort.java

@@ -0,0 +1,66 @@
+/**
+ * Merge sort an integer arrays
+ * 
+ * Tags: Sort
+ */
+class MergeSort {
+    public static void main(String[] args) {
+        MergeSort m = new MergeSort();
+        int[] A = { 1, 4, 2, 8, 5 };
+        for (int n : A) System.out.print(n + ",");
+        System.out.println();
+        m.sort(A, 0, A.length - 1);
+        for (int n : A) System.out.print(n + ",");
+    }
+    
+    /**
+     * If range exists
+     * Get middle index
+     * Sort first half, from low to middle
+     * Sort second half, from middle + 1 to high
+     * Merge these two halves
+     */
+    public void sort(int[] A, int low, int high) {
+        if (low < high) {
+            int middle = low + (high - low) / 2;
+            sort(A, low, middle);
+            sort(A, middle + 1, high);
+            merge(A, low, middle, high);
+        }
+    }
+    
+    /**
+     * Copy items from low to high to a helper array
+     * Init 2 pointers 
+     * Compare value of 2 pointers, and overwrite original array, and move on
+     * Stop when left reach middle or right reach high
+     * Right side already in original array, so copy items remain in left side
+     * to original array
+     */
+    private void merge(int[] A, int low, int middle, int high) {
+        int[] helper = new int[A.length];
+        for (int i = low; i <= high; i++) {
+            helper[i] = A[i];
+        }
+        
+        int left = low;
+        int right = middle + 1;
+        int curIdx = low;
+        
+        while (left <= middle && right <= high) {
+            if (helper[left] <= helper[right]) {
+                A[curIdx] = helper[left];
+                left++;
+            } else {
+                A[curIdx] = helper[right];
+                right++;
+            }
+            curIdx++;
+        }
+        
+        int remain = middle - left; // items remain in left part
+        for (int i = 0; i <= remain; i++) {
+            A[curIdx + i] = helper[left + i]; // move to origin array
+        }
+    }
+}