Add day23

Author: MadhavBahl

BONUS/README.md

@@ -48,9 +48,15 @@ In case you just want to contribute a question and not code, there is no need to
 
 - To be added
 
-### [2. Recursion](./Recursion/README.md)
-
-- To be added
+### [2. Recursion](./Recursion/)
+
+1. [WAP to calculate `a` raised to the power `n` using recursion](./Recursion/Power/)
+2. [WAP to calculate the factorial of a given number usinig recursion](./Recursion/Factorial/)
+3. [WAP to calculate the product of elements in the given array using recursion](./Recursion/ArrayProduct/)
+4. [WAP to calculate the sum of elements in a given integer array using recursion](./Recursion/ArraySum/)
+5. [WAP to find the nth number in Fibonacci Series using recursion](./Recursion/Fibonacci/)
+6. [WAP to reverse a string using recursion](./Recursion/ReverseString/)
+7. [WAP to reverse an integer array using recursion](./Recursion/ReverseArray/)
 
 ### [3. Array](./Arrays/README.md)
 

BONUS/Recursion/ReverseArray/README.md

@@ -0,0 +1,108 @@
+# Reverse Array
+
+WAP to reverse an array using recursion
+
+**Example**
+
+```
+input: [1, 2, 3, 4]
+output: [4, 3, 2, 1]
+```
+
+## Solution
+
+### [JavaScript Implementation (method 1)](./reverseArray.js)
+
+```js
+/** 
+ * Reverse an array using recursion
+ * Method 1
+ * Implemented by MadhavBahlMD
+ * @date 18/01/2019
+ */
+
+function reverseArray (arr) {
+    if (arr.length === 1) return arr;
+    return reverseArray(arr.slice(1, arr.length)).concat([arr[0]]);
+}
+
+console.log (reverseArray([1, 2, 3, 4]));
+```
+
+### [JavaScript Implementation (method 2)](./reverseArray.js)
+
+```js
+/** 
+ * Reverse an array using recursion
+ * Method 1
+ * Implemented by MadhavBahlMD
+ * @date 18/01/2019
+ */
+
+function reverseArray (arr, startIndex, endIndex) {
+    if (startIndex >= endIndex) return arr;
+
+    let temp = arr[startIndex];
+    arr[startIndex] = arr[endIndex];
+    arr[endIndex] = temp;
+
+    reverseArray (arr, startIndex+1, endIndex-1);
+}
+
+let arr = [1, 2, 3, 4];
+console.log (`Original Array: ${arr}`);
+reverseArray (arr, 0, arr.length-1);
+console.log (`Reversed Array: ${arr}`);
+```
+
+### [Java Implementation](./ReverseArray.java)
+
+```java
+import java.util.Scanner;
+
+/**
+ * Reverse an array using recursion
+ * @author MadhavBahlMD
+ * @date 18/01/2019
+ */
+
+public class ReverseArray {
+    public static void reverse (int[] arr, int startIndex, int endIndex) {
+        if (startIndex >= endIndex) return;
+
+        // Swap equidistant elements from start and end
+        int temp = arr[startIndex];
+        arr[startIndex] = arr[endIndex];
+        arr[endIndex] = temp;
+
+        reverse (arr, startIndex+1, endIndex-1);
+    }
+
+    public static void main(String[] args) {
+        Scanner input = new Scanner (System.in);
+        System.out.println("/* ===== Sum of Array elements using recursion ===== */");
+
+        // Input the array
+        System.out.print("\nEnter the number of elements in the array: ");
+        int n = input.nextInt();
+        int arr[] = new int[n];
+        for (int i=0; i<n; i++) {
+            System.out.print("Enter arr[" + i + "]: ");
+            arr[i] = input.nextInt();
+        }
+
+        // Print the original array
+        System.out.print("Original Array: ");
+        for (int i=0; i<arr.length; i++)
+            System.out.print(arr[i] +  " ");
+
+        // Reverse the array
+        reverse(arr, 0, arr.length-1);
+
+        // Print the reversed array
+        System.out.print("\nReversed Array: ");
+        for (int i=0; i<arr.length; i++)
+            System.out.print(arr[i] +  " ");
+    }
+}
+```

BONUS/Recursion/ReverseArray/ReverseArray.java

@@ -0,0 +1,47 @@
+import java.util.Scanner;
+
+/**
+ * Reverse an array using recursion
+ * @author MadhavBahlMD
+ * @date 18/01/2019
+ */
+
+public class ReverseArray {
+    public static void reverse (int[] arr, int startIndex, int endIndex) {
+        if (startIndex >= endIndex) return;
+
+        // Swap equidistant elements from start and end
+        int temp = arr[startIndex];
+        arr[startIndex] = arr[endIndex];
+        arr[endIndex] = temp;
+
+        reverse (arr, startIndex+1, endIndex-1);
+    }
+
+    public static void main(String[] args) {
+        Scanner input = new Scanner (System.in);
+        System.out.println("/* ===== Sum of Array elements using recursion ===== */");
+
+        // Input the array
+        System.out.print("\nEnter the number of elements in the array: ");
+        int n = input.nextInt();
+        int arr[] = new int[n];
+        for (int i=0; i<n; i++) {
+            System.out.print("Enter arr[" + i + "]: ");
+            arr[i] = input.nextInt();
+        }
+
+        // Print the original array
+        System.out.print("Original Array: ");
+        for (int i=0; i<arr.length; i++)
+            System.out.print(arr[i] +  " ");
+
+        // Reverse the array
+        reverse(arr, 0, arr.length-1);
+
+        // Print the reversed array
+        System.out.print("\nReversed Array: ");
+        for (int i=0; i<arr.length; i++)
+            System.out.print(arr[i] +  " ");
+    }
+}

BONUS/Recursion/ReverseArray/reverseArray.js

@@ -0,0 +1,13 @@
+/** 
+ * Reverse an array using recursion
+ * Method 1
+ * Implemented by MadhavBahlMD
+ * @date 18/01/2019
+ */
+
+function reverseArray (arr) {
+    if (arr.length === 1) return arr;
+    return reverseArray(arr.slice(1, arr.length)).concat([arr[0]]);
+}
+
+console.log (reverseArray([1, 2, 3, 4]));

BONUS/Recursion/ReverseArray/reverseArray2.js

@@ -0,0 +1,21 @@
+/** 
+ * Reverse an array using recursion
+ * Method 1
+ * Implemented by MadhavBahlMD
+ * @date 18/01/2019
+ */
+
+function reverseArray (arr, startIndex, endIndex) {
+    if (startIndex >= endIndex) return arr;
+
+    let temp = arr[startIndex];
+    arr[startIndex] = arr[endIndex];
+    arr[endIndex] = temp;
+
+    reverseArray (arr, startIndex+1, endIndex-1);
+}
+
+let arr = [1, 2, 3, 4];
+console.log (`Original Array: ${arr}`);
+reverseArray (arr, 0, arr.length-1);
+console.log (`Reversed Array: ${arr}`);

README.md

@@ -46,6 +46,7 @@ Read [CONTRIBUTING.md](./CONTRIBUTING.md) for contribution guidelines.
 20. [Day 20 -- Array Partition](./day20) -- [http://codetoexpress.tech/dc/day20/](http://codetoexpress.tech/dc/day20/)
 21. [Day 21 -- Pair Sum N and Max Subarray Sum](./day21) -- [http://codetoexpress.tech/dc/day21/](http://codetoexpress.tech/dc/day21/)
 22. [Day 22 -- Common Elements Search](./day22) -- [http://codetoexpress.tech/dc/day22/](http://codetoexpress.tech/dc/day22/)
+23. [Day 23 -- Combination Sum](./day23) -- [http://codetoexpress.tech/dc/day23/](http://codetoexpress.tech/dc/day23/)
 
 ## [More Problems](./BONUS/README.md)
 
@@ -93,6 +94,15 @@ In case you just want to contribute a question and not code, there is no need to
 7. [**Linked Lists**](./BONUS/LinkedLists/README.md)
 8. [**Trees**](./BONUS/Trees/README.md)
 9. [**Graphs**](./BONUS/Graphs/README.md)
+10. [**Misc Questions**](./BONUS/Misc/README.md)
+
+## EXTRA - Past Interview Questions
+
+Here are some of the questions asked previously in major companies like Google, Amazon, Facebook, Microsoft etc.
+
+```
+To Be Added
+```
 
 ## Timeline
 

day23/JavaScript/combinationSum.js

@@ -0,0 +1 @@
+// To Be Added

day23/README.md

@@ -0,0 +1,41 @@
+![cover](./cover.png)
+
+# Day 23 - Array Series Part 6: Combination Sum
+
+**Question Source -- [Geeks4Geeks](https://www.geeksforgeeks.org/combinational-sum/)**
+
+**Question** -- Given a set of integers, and a number, find all unique combinations in the integer array whose sum equals the given number.
+
+**Example**
+
+```
+input: arr = [2,3,6,7], num = 7
+output: Solution set -
+[
+    [7],
+    [2,2,3]
+]
+
+input: arr = [2, 4, 6, 8], num = 8
+output: Solution set -
+[
+    [2, 2, 2, 2]
+    [2, 2, 4]
+    [2, 6]
+    [4, 4]
+    [8]
+]
+```
+
+![ques](./ques.png)
+
+## Solution
+
+## JavaScript Implementation
+
+### [Soluion](./JavaScript/combinationSum.js)
+
+```
+to be added
+```
+