Add pigeonhole sort (fixes #2992) (#3013)

src/main/java/com/thealgorithms/sorts/PigeonholeSort.java

@@ -0,0 +1,54 @@
+package com.thealgorithms.sorts;
+
+import java.util.*;
+import static com.thealgorithms.sorts.SortUtils.*;
+
+public class PigeonholeSort {
+    /*
+        This code implements the pigeonhole sort algorithm for the integer array,
+        but we can also implement this for string arrays too.
+        See https://www.geeksforgeeks.org/pigeonhole-sort/
+    */
+    void sort(Integer[] array){
+        int maxElement = array[0];
+        for (int element: array) {
+            if (element > maxElement) maxElement = element;
+        }
+
+        int numOfPigeonholes = 1 + maxElement;
+        ArrayList<Integer>[] pigeonHole =  new ArrayList[numOfPigeonholes];
+
+        for (int k=0; k<numOfPigeonholes; k++) {
+            pigeonHole[k] = new ArrayList<>();
+        }
+
+        for (int t: array) {
+            pigeonHole[t].add(t);
+        }
+
+        int k=0;
+        for (ArrayList<Integer> ph: pigeonHole) {
+            for (int elements: ph) {
+                array[k]=elements;
+                k=k+1;
+            }
+        }
+    }
+
+    public static void main(String[] args)
+    {
+        PigeonholeSort pigeonholeSort = new PigeonholeSort();
+        Integer[] arr = { 8, 3, 2, 7, 4, 6, 8 };
+
+        System.out.print("Unsorted order is : ");
+        print(arr);
+
+        pigeonholeSort.sort(arr);
+        
+        System.out.print("Sorted order is : ");
+        for (int i = 0; i < arr.length; i++) {
+            assert (arr[i]) <= (arr[i+1]);
+        }
+        print(arr);
+    }
+}