Decrypt a string encrypted by repeating i-th character i times

Given an encrypted string str and the encryption algorithm, the task is to decrypt the string. The encryption algorithm is as follows: 
The 1^st character of the string will be repeated once in the encrypted string, the 2^nd character will be repeated twice, …, n^th character will be repeated n times. For example, the string “abcd” will be encrypted as “abbcccdddd”.

Examples:

Input: str = “geeeeekkkksssss” 
Output: geeks

Input: str = “abbcccdddd” 
Output: abcd 

Approach: Initialize i = 0 and print str[i]. 
Update i = i + 1 and print str[i], then update i = i + 2 and print str[i] and so on while i < length(str).

Below is the implementation of the above approach:  

C++

// C++ implementation of the approach
#include <bits/stdc++.h>
using namespace std;

// Function to return the decrypted string
string decryptString(string str, int n)
{

    // Initial jump will be 1
    int i = 0, jump = 1;
    string decryptedStr = "";

    while (i < n) {
        decryptedStr += str[i];
        i += jump;

        // Increment jump by 1 with every character
        jump++;
    }

    return decryptedStr;
}

// Driver code
int main()
{
    string str = "geeeeekkkksssss";
    int n = str.length();
    cout << decryptString(str, n);

    return 0;
}

Java

// Java implementation of the approach
class GFG
{

// Function to return the decrypted string
static String decryptString(String str, int n)
{

    // Initial jump will be 1
    int i = 0, jump = 1;
    String decryptedStr = "";

    while (i < n)
    {
        decryptedStr += str.charAt(i);
        i += jump;

        // Increment jump by 1 with every character
        jump++;
    }

    return decryptedStr;
}

// Driver code
public static void main(String[] args)
{
    String str = "geeeeekkkksssss";
    int n = str.length();
    System.out.println(decryptString(str, n));
}
}

// This code is contributed by Code_Mech

Python3

# Python 3 implementation of the approach

# Function to return the decrypted string
def decryptString(str, n):

    # Initial jump will be 1
    i = 0
    jump = 1
    decryptedStr = ""

    while (i < n):
        decryptedStr += str[i];
        i += jump

        # Increment jump by 1 with
        # every character
        jump += 1

    return decryptedStr

# Driver code
if __name__ == '__main__':
    str = "geeeeekkkksssss"
    n = len(str)
    print(decryptString(str, n))

# This code is contributed by
# Surendra_Gangwar

C#

// C# implementation of the approach
using System;

class GFG
{

// Function to return the decrypted string
static string decryptString(string str, int n)
{

    // Initial jump will be 1
    int i = 0, jump = 1;
    string decryptedStr = "";

    while (i < n)
    {
        decryptedStr += str[i];
        i += jump;

        // Increment jump by 1 with every character
        jump++;
    }

    return decryptedStr;
}

// Driver code
public static void Main()
{
    string str = "geeeeekkkksssss";
    int n = str.Length;
    Console.Write(decryptString(str, n));
}
}

// This code is contributed by ita_c

PHP

<?php
// PHP implementation of the approach

// Function to return the decrypted string
function decryptString($str, $n)
{

    // Initial jump will be 1
    $i = 0 ;
    $jump = 1 ;
    $decryptedStr = "";

    while ($i < $n)
    {
        $decryptedStr .= $str[$i];
        $i += $jump;

        // Increment jump by 1 with
        // every character
        $jump++;
    }
    return $decryptedStr;
}

// Driver code
$str = "geeeeekkkksssss";
$n = strlen($str);
echo decryptString($str, $n);

// This code is contributed by Ryuga
?>

Javascript

<script>
// Javascript implementation of the approach

// Function to return the decrypted string
function decryptString(str,n)
{
    // Initial jump will be 1
    let i = 0, jump = 1;
    let decryptedStr = "";

    while (i < n)
    {
        decryptedStr += str[i];
        i += jump;

        // Increment jump by 1 with every character
        jump++;
    }

    return decryptedStr;
}

// Driver code
let str = "geeeeekkkksssss";
let n = str.length;
document.write(decryptString(str, n));


// This code is contributed by rag2127
</script>

Output

geeks

Time Complexity: O(?n), where n is the length of the given string.
Auxiliary Space: O(?n), where n is the length of the given string.