# Problem Solving Using Recursion in C++

If you think recursively, you can solve many problems using recursion.

The preceding sections presented two classic recursion examples. All recursive functions have the following characteristics:

• The function is implemented using an if-else or a switch statement that leads to if-else different cases.
• One or more base cases(the simplest case) are used to stop recursion. base cases
• Every recursive call reduces the original problem, bringing it increasingly closer to a reduction base case until it becomes that case.

In general, to solve a problem using recursion, you break it into subproblems. If a subproblem resembles the original problem, you can apply the same approach to solve the subproblem recursively. This subproblem is almost the same as the original problem in nature with a smaller size.

Recursion is everywhere. It is fun to think recursively. Consider drinking coffee. You may think recursively describe the procedure recursively as follows:

void drinkCoffee(Cup& cup)

{

if (!cup.isEmpty())

{

cup.takeOneSip(); // Take one sip drinkCoffee(cup);

}

} Assume cup is an object for a cup of coffee with the instance functions isEmpty() and takeOneSip(). You can break the problem into two subproblems: one is to drink one sip of coffee and the other is to drink the rest of the coffee in the cup. The second problem is the same as the original problem but smaller in size. The base case for the problem is when cup is empty.

Let us consider a simple problem of printing a message for n times. You can break the problem into two subproblems: one is to print the message one time and the other is to print the message n – 1 times. The second problem is the same as the original problem with a smaller size. The base case for the problem is n == 0. You can solve this problem using recursion as follows:

void nPrintln(const string& message, int times)

{

if (times >= 1)

{

cout << message << endl;

nPrintln(message, times – 1);

} // The base case is times == 0}

}

Note that the fib function in Listing 17.2 returns a value to its caller, but the nPrintln func­tion is void and does not return a value to its caller.

Many of the problems presented in the early chapters can be solved using recursion if  you think recursively.

Consider the palindrome problem in Listing 5.16, TestPalindrome.cpp.

Recall that a string is a palindrome if it reads the same from the left and from the right. For example, mom and dad are palindromes, but uncle and aunt are not. The problem to check whether a string is a palindrome can be divided into two subproblems:

• Check whether the first character and the last character of the string are equal.
• Ignore these two end characters and check whether the rest of the substring is a palindrome.

The second subproblem is the same as the original problem with a smaller size. There are two base cases: (1) the two end characters are not the same; (2) the string size is 0 or 1. In case 1, the string is not a palindrome; and in case 2, the string is a palindrome. The recursive function for this problem can be implemented in Listing 17.3.

Listing 17.3 RecursivePalindrome.cpp

1 #include <iostream>
2
#include <string>
3
using namespace std;
4
5
bool isPalindrome(const string& s)
6 {
7
if (s.size() <= 1) // Base case
8        return true;
9
else if (s != s[s.size() – 1]) // Base case
10       return false;
11
else
12       return isPalindrome(s.substr(1, s.size() – 2));
13 }
14
15
int main()
16 {
17    cout <<
“Enter a string: “;
18    string s;
19    getline(cin, s);
20
21
if (isPalindrome(s))
22       cout << s <<
” is a palindrome” << endl;
23
else
24       cout << s << ” is not a palindrome” << endl;
25
26
return 0;
27 } The isPalindrome function checks whether the size of the string is less than or equal to 1 (line 7). If so, the string is a palindrome. The function checks whether the first and the last elements of the string are the same (line 9). If not, the string is not a palindrome. Otherwise, obtain a substring of s using s.substr(1, s.size() – 2) and recursively invoke isPalindrome with the new string (line 12).

Source: Liang Y. Daniel (2013), Introduction to programming with C++, Pearson; 3rd edition.