1. CS 211 Object Oriented Programming
Tail Recursion

2. Tail Recursion
A method is tail recursive if the last action of the recursive method is the recursive call
Recursion puts frame on the stack
the additional overhead required by non-tail recursion could be costly for large inputs

3. Non Tail Recursion //Factorial public static int recurciveFact(int n){
if(n == 0)
return 1; }
else
return n*recurciveFact(n-1);

4. Simulation recurciveFact(4) = 4*recurciveFact(3)
=1*1 =1
= 2*1 = 2
= 3*2 = 6
= 4*6 = 24

5. Recursion in Stack
Factorial :

6. Tail Recursion – Factorial
public static int tailrecurciveFact(int n) {
return factorial(n,1); }
private static int factorial(int n,int accum)
if(n==0)
return accum;
else{
return factorial(n-1,n*accum);

7. Simulation tailrecurciveFact(4) = factorial(4,1)
factorial(4,1) = factorial(3,4*1)
factorial(3,4*1) = factorial(2,4*1*3)
factorial(2,4*1*3) = factorial(1,4*1*3*2)
factorial(1,4*1*3*2)
= factorial(0,4*1*3*2*1)
= 24
factorial(2,4*1*3) = 24
factorial(3,4*1) = 24
factorial(4,1) = 24
tailrecurciveFact(4) = 24;

8. Tail Recursion - Fibonacci
public int fibonacci(int n) {
if(n == 0)
return 0;
else if(n == 1)
return 1;
else
return fibonacci(n - 1) +
fibonacci(n - 2); }

9. Simulation //Fibonacci fibonacci(5) = fibonacci(4) + fibonacci(3)

10. Non-Tail Recursion public String rev(String w) {
if(w == null || w.equals(""))
return w;
else
return rev(w.substring(1, w.length()))
+ w.substring(0,1); }

11. Tail Recursion //”hook” public String rev(String w) {
return tailRev(w, ""); }
//tail recursion
public String tRev(String w, String res) {
if(w==null || w.equals(""))
return res;
else
return tRev(w.substring(1, w.length()),
w.charAt(0) + res);

12. Questions?