Added recursion

Author: Prashant Jain

src/in/knowledgegate/dsa/recursion/Factorial.java

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+package in.knowledgegate.dsa.recursion;
+
+/**
+ * Factorial
+ * n! = n X (n-1) X ..... X 1
+ * F(0) = 1
+ * F(1) = 1
+ * F(2) = 2
+ * F(3) = 6 = 3 * 2
+ * F(n) = n * F(n-1), n >= 1
+ */
+public class Factorial {
+  public int fact(int n) {
+    if (n == 0 || n == 1) return 1;
+    return n * fact(n-1);
+  }
+}

src/in/knowledgegate/dsa/recursion/Fibonacci.java

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+package in.knowledgegate.dsa.recursion;
+
+/**
+ * The Fibonacci numbers, commonly denoted F(n)
+ * form a sequence, called the Fibonacci sequence,
+ * such that each number is the sum of the two
+ * preceding ones, starting from 0 and 1. That is,
+ *
+ * F(0) = 0, F(1) = 1
+ * F(n) = F(n - 1) + F(n - 2), for n > 1.
+ * Given n, calculate F(n).
+ *
+ *
+ * Example 1:
+ * Input: n = 2
+ * Output: 1
+ * Explanation: F(2) = F(1) + F(0) = 1 + 0 = 1.
+ *
+ * Example 2:
+ * Input: n = 3
+ * Output: 2
+ * Explanation: F(3) = F(2) + F(1) = 1 + 1 = 2.
+ *
+ * Example 3:
+ * Input: n = 4
+ * Output: 3
+ * Explanation: F(4) = F(3) + F(2) = 2 + 1 = 3.
+ *
+ *
+ * Constraints:
+ * 0 <= n <= 30
+ */
+public class Fibonacci {
+  public int fibRecursive(int n) {
+    if (n == 0 || n == 1) return n;
+    return fibRecursive(n - 1) +
+        fibIterative( n - 2);
+  }
+
+  public int fibIterative(int n) {
+    if (n == 0 || n == 1) return n;
+    int first = 0, second = 1;
+    for (int i = 1; i < n; i++) {
+      int next = first + second;
+      first = second;
+      second = next;
+    }
+    return second;
+  }
+}