merge Sort

Author: yuzhangcmu

sort/MergeSort.java

@@ -0,0 +1,123 @@
+package Algorithms.sort;
+
+/********************************************************
+ * 
+ * 08-722 Data Structures for Application Programmers
+ * Lecture 14 Advanced Sorting
+ * 
+ * Naive version of Merge Sort
+ * 
+ *********************************************************/
+import java.util.Arrays;
+
+public class MergeSort {
+
+    private static final int SIZE = 10000;
+
+    public static int[] mergeSort(int[] data) {
+        // parameter valid judge.
+        if (data == null) {
+            return null;
+        }
+
+        // the base case.
+        int len = data.length;
+        if (len <= 1) {
+            return data;
+        }
+
+        // divide into two arrays.
+        // create left half and right half.
+        int left[] = new int[len/2];
+        int right[] = new int[len - len/2];
+
+        System.arraycopy(data, 0, left, 0, len/2);
+        System.arraycopy(data, len/2, right, 0, len - len/2);
+
+        // call itself to sort left half
+        left = mergeSort(left);
+        right = mergeSort(right);
+        
+        return merge(left, right);
+    }
+
+    // Precondition: two input arrays are sorted and they are not null.
+    private static int[] merge(int[] left, int[] right) {
+        int len = left.length + right.length;
+
+        int[] ret = new int[len];
+
+        int leftPoint = 0;
+        int rightPoint = 0;
+
+        int cur = 0;
+        while (leftPoint < left.length && rightPoint < right.length) {
+            if (left[leftPoint] < right[rightPoint]) {
+                ret[cur++] = left[leftPoint++];
+            } else {
+                ret[cur++] = right[rightPoint++];
+            }
+        }
+
+        if (leftPoint >= left.length) {
+            while (rightPoint < right.length) {
+                ret[cur++] = right[rightPoint++];
+            }
+        } else {
+            while (leftPoint < left.length) {
+                ret[cur++] = left[leftPoint++];
+            }
+        }
+
+        return ret;
+    }
+
+    public static void mergeSort2(int[] data) {
+        // parameter valid judge.
+        if (data == null) {
+            return null;
+        }
+
+        // the base case.
+        int len = data.length;
+        if (len <= 1) {
+            return data;
+        }
+
+        // divide into two arrays.
+        // create left half and right half.
+        int left[] = new int[len/2];
+        int right[] = new int[len - len/2];
+
+        System.arraycopy(data, 0, left, 0, len/2);
+        System.arraycopy(data, len/2, right, 0, len - len/2);
+
+        // call itself to sort left half
+        left = mergeSort(left);
+        right = mergeSort(right);
+        
+        return merge(left, right);
+    }
+
+    public static void main(String[] args) {
+        int[] a = new int[SIZE];
+        for (int i = 0; i < SIZE; i++)
+            a[i] = (int) (Math.random() * SIZE);
+
+        //mergeSort(a);
+
+        int[] test = { 42, 12, 89, 27, 94, 63, 3, 78 };
+        System.out.println(Arrays.toString(mergeSort(test)));
+
+        // test merge method
+        int[] left = { 12, 42, 63, 89 };
+        int[] right = { 3, 27, 78, 94 };
+        System.out.println(Arrays.toString(merge(left, right)));
+
+        // test merge method
+        int[] left2 = {};
+        int[] right2 = { 3, 27, 78, 94 };
+        System.out.println(Arrays.toString(merge(left2, right2)));
+    }
+
+}