segregate 0s 1 2s : done

Author: Ram swaroop

src/me/ramswaroop/arrays/Segregate0s1sAnd2s.java

@@ -0,0 +1,65 @@
+package me.ramswaroop.arrays;
+
+import java.util.Arrays;
+
+/**
+ * Created by IntelliJ IDEA.
+ *
+ * @author: ramswaroop
+ * @date: 8/18/15
+ * @time: 8:38 PM
+ */
+public class Segregate0s1sAnd2s {
+
+    /**
+     * Segregates an array {@param a} consisting of only 0s, 1s and 2s. Based on 
+     * Dutch National Flag (DNF) problem {@see: http://www.csse.monash.edu.au/~lloyd/tildeAlgDS/Sort/Flag/}.
+     * 
+     * @param a
+     */
+    public static void segregate0s1sAnd2s(int[] a) {
+        // assume low points to 0 and mid to 1 and high to 2
+        int low = 0, mid = 0, high = a.length - 1;
+        
+        while (mid <= high) {
+            switch (a[mid]) {
+                case 0: // mid points to 0 but it should point to 1 so swap it with low
+                    swap(a, low, mid);
+                    low++;
+                    mid++;
+                    break;
+                case 1: // mid points to 1 which is correct acc. to our assumption so proceed
+                    mid++;
+                    break;
+                case 2: // mid points to 2 instead of 1 so swap it with high
+                    swap(a, mid, high);
+                    high--;
+                    break;
+            }
+        }
+    }
+
+    private static void swap(int[] a, int index1, int index2) {
+        int temp = a[index1];
+        a[index1] = a[index2];
+        a[index2] = temp;
+    }
+
+    public static void main(String a[]) {
+        int[] ar = new int[]{0, 1, 2, 0, 1, 2};
+        segregate0s1sAnd2s(ar);
+        System.out.println(Arrays.toString(ar));
+
+        int[] ar1 = new int[]{0, 2, 1, 1, 2, 0};
+        segregate0s1sAnd2s(ar1);
+        System.out.println(Arrays.toString(ar1));
+
+        int[] ar2 = new int[]{0, 1, 2};
+        segregate0s1sAnd2s(ar2);
+        System.out.println(Arrays.toString(ar2));
+
+        int[] ar3 = new int[]{2, 1, 0, 2, 1, 0};
+        segregate0s1sAnd2s(ar3);
+        System.out.println(Arrays.toString(ar3));
+    }
+}