C++ Program To Print Reverse of a String Using Recursion
Write a recursive function to print the reverse of a given string.
Code:
C++
Output:
skeeG rof skeeG
Explanation: Recursive function (reverse) takes string pointer (str) as input and calls itself with next location to passed pointer (str+1). Recursion continues this way when the pointer reaches ‘\0’, all functions accumulated in stack print char at passed location (str) and return one by one.
Time Complexity: O(n^2) as substr() method has a time complexity of O(k) where k is the size of the returned string. So for every recursive call, we are reducing the size of the string by one, which leads to a series like (k-1)+(k-2)+…+1 = k*(k-1)/2 = O(k^2) = O(n^2)
See Reverse a string for other methods to reverse string.
Auxiliary Space: O(n)
Efficient Approach:
We can store each character in recursive stack and then can print while coming back as shown in the below code:
C++
// C++ program to reverse a string using recursion #include <bits/stdc++.h> using namespace std; /* Function to print reverse of the passed string */void reverse(char *str, int index, int n) { if(index == n) // return if we reached at last index or at the end of the string { return; } char temp = str[index]; // storing each character starting from index 0 in function call stack; reverse(str, index+1, n); // calling recursive function by increasing index everytime cout << temp; // printing each stored character while recurring back } /* Driver program to test above function */int main() { char a[] = "Geeks for Geeks"; int n = sizeof(a) / sizeof(a[0]); reverse(a, 0, n); return 0; } |
Output:
skeeG rof skeeG
Time Complexity: O(n) where n is size of the string
Auxiliary Space: O(n) where n is the size of string, which will be used in the form of function call stack of recursion.
Please suggest if someone has a better solution that is more efficient in terms of space and time.
