chore: adding interview questions

Author: rupeshtiwari

README.md

@@ -650,14 +650,17 @@ abstract data type (ADT) - ADT is defined as a user point of view of a data type
 
 ![x to the power n using optimized recursion with multiple subproblems](https://i.imgur.com/Ym3qjhj.png)
 
+- [Question](https://codepen.io/roopkt/pen/XWMxvKg?editors=0010)
+- [Answer](https://codepen.io/roopkt/pen/oNZaKLy?editors=0010)
+
 #### Calculate Modular Exponentiation using recursion
 
 Modular Exponentiation is the remainder dividing up on `Pow(x,n)` by `M`.
 
 ![Modular Exponentiation is the remainder dividing up on `Pow(x,n)` by `M`](https://i.imgur.com/Y3GkhRT.png)
 
-- [Question](https://codepen.io/roopkt/pen/XWMxvKg?editors=0010)
-- [Answer](https://codepen.io/roopkt/pen/oNZaKLy?editors=0010)
+- [Question](https://codepen.io/roopkt/pen/GRWYVEa?editors=0010)
+- [Answer](https://codepen.io/roopkt/pen/gOmBVWJ?editors=0010)
 
 ## References
 

src/interview-questions/recursion/modular-exponentiation.mjs

@@ -0,0 +1,16 @@
+/**
+Time complexity O(logN)
+
+*/
+
+function mod(x, n, m) {
+  if (n === 0) return 1;
+  if (n % 2 === 0) {
+    const y = mod(x, n / 2, m);
+    return (y * y) % m;
+  } else {
+    const p = (x % m) * mod(x, n - 1, m);
+
+    return p % m;
+  }
+}