Counting Sort

Author: aveerepalli

src/com/deepak/algorithms/Searching/BinarySearch.java

@@ -43,7 +43,7 @@ public static void main(String[] args) {
 	 *  
 	 * 2. Worst Case : What is the most number of comparisons needed to find the item?
 	 * 	=> Worst case is when search term is not at all in the array
-	 * 	=> If our array is of size N, we need N comparisons for worst case
+	 * 	=> If our array is of size N, we need log(2)N comparisons for worst case
 	 * 
 	 * 3. Average Case : On an Average, how many comparisons are needed to find the element in the array?
 	 * 	=> On an average, search term is anywhere in the list

src/com/deepak/algorithms/Sorting/CountingSort.java

@@ -20,7 +20,58 @@ public static void main(String[] args) {
 		performCountingSort(valuesToBeSorted);
 	}
 
-	private static void performCountingSort(int[] values) {
+	/**
+	 * Counting sort is a sorting technique based on keys between a specific range.
+	 * It works by counting the number of objects having distinct key values (kind of hashing).
+	 * Then doing some arithmetic to calculate the position of each object in the output sequence.
+	 * <p>Notes:
+	 * 	1. Counting sort is efficient if the range of input data is not significantly greater than the number of objects to be sorted.
+	 * 		Consider the situation where the input sequence is between range 1 to 10K and the data is 10, 5, 10K, 5K.
+	 	2. It is not a comparison based sorting. It running time complexity is O(n) with space proportional to the range of data.
+	 	3. It is often used as a sub-routine to another sorting algorithm like radix sort.
+	 	4. Counting sort uses a partial hashing to count the occurrence of the data object in O(1).
+	 	5. Counting sort can be extended to work for negative inputs also.
+	 	6. This type of integer sorting algorithms are usually designed to work in either the pointer machine or random access machine models of computing
+	 	</p>
+	 */
+    private static void performCountingSort(int[] values) {
+
+
+			int n = values.length;
+
+			// The output character array that will have sorted arr
+			int output[] = new int[n];
+
+			// Create a count array to store count of inidividul
+			// characters and initialize count array as 0
+			int count[] = new int[256];
+			for (int i=0; i<256; ++i)
+				count[i] = 0;
+
+			// store count of each character
+			for (int i=0; i<n; ++i)
+				++count[values[i]];
+
+			// Change count[i] so that count[i] now contains actual
+			// position of this character in output array
+			for (int i=1; i<=255; ++i)
+				count[i] += count[i-1];
+
+			// Build the output character array
+			for (int i = 0; i<n; ++i)
+			{
+				output[count[values[i]]-1] = values[i];
+				--count[values[i]];
+			}
+
+			// Copy the output array to arr, so that arr now
+			// contains sorted characters
+			for (int i = 0; i<n; ++i)
+				values[i] = output[i];
+
+		System.out.print("Sorted  array is ");
+		for (int i=0; i<values.length; ++i)
+			System.out.print(values[i]+",");
 
 	}