Decrypt a string according to given rules

Given an encrypted string str, the task is to decrypt the given string when the encryption rules are as follows:

  1. Start with the first character of the original string.
  2. In every odd step, append the next character to it.
  3. In every even step, prepend the next character to the encrypted string so far.

For example, if str = “geeks” then the encrypted string will be,
g -> ge -> ege -> egek -> segek

Examples:



Input: str = “segosegekfrek”
Output: geeksforgeeks

Input: str = “vrstie”
Output: strive

Approach: The steps that are given to encrypt the string can be followed in reverse order to obtain the original string. There will be two conditions to obtain the original string:

  • If the string is of odd length, in odd step add the characters from the back in the resultant string, else add from the front.
  • If the string is of even length, in odd step add characters from the front and add characters from back in even step.

The reverse of the resultant string thus obtained is the original string which was encrypted.

Below is the implementation of the above approach:

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// C++ program to decrypt the original string
#include <bits/stdc++.h>
using namespace std;
  
// Function to return the original string 
// after decryption
string decrypt(string s, int l)
{
    // Stores the decrypted string
    string ans = "";
  
    // If length is odd
    if (l % 2) {
  
        // Step counter
        int cnt = 0;
  
        // Starting and ending index
        int indl = 0, indr = l - 1;
  
        // Iterate till all characters 
        // are decrypted
        while (ans.size() != l) {
  
            // Even step
            if (cnt % 2 == 0)
                ans += s[indl++];
  
            // Odd step
            else
                ans += s[indr--];
  
            cnt++;
        }
    }
  
    // If length is even
    else {
  
        // Step counter
        int cnt = 0;
  
        // Starting and ending index
        int indl = 0, indr = l - 1;
        while (ans.size() != l) {
  
            // Even step
            if (cnt % 2 == 0)
                ans += s[indr--];
  
            // Odd step
            else
                ans += s[indl++];
  
            cnt++;
        }
    }
  
    // Reverse the decrypted string
    reverse(ans.begin(), ans.end());
  
    return ans;
}
  
// Driver Code
int main()
{
    string s = "segosegekfrek";
    int l = s.length();
    cout << decrypt(s, l);
  
    return 0;
}

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Output:

geeksforgeeks


My Personal Notes arrow_drop_up

Striver(underscore)79 at Codechef and codeforces D

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