Decrypt message from given code by replacing all * with prefix values of encoded string

Given a string str of length of N that is in the encoded form with alphabets and * . The task is to find the string from which it was generated. The required string can be generated from the encoded string by replacing all the * with the prefix values of the encoded string. 

Examples:

Input: str =  ab*c*d
Output: "ababcababcd"
Explanation: For the first occurrence of  "*", "ab" is the prefix. So '*' is replaced by "ab" resulting the string  to be "ababc*d"  and for the next '*' the prefix is "ababc". So the string will now change from "ababc*d" to "ababcababcd".

Input :  str = "z*z*z"
Output:  zzzzzzz

Approach: The solution is based on greedy approach. Follow the steps mentioned below to solve the problem:

• Consider an empty string result.
• Iterate over the given coded string.
  • if the current character in the string is not "*" then add the current character to the result.
  • Otherwise, if the current character is "*", add the result string formed till now with itself.
• Return the result.

Below is the implementation of the given approach.

// C++ code to implement above approach
#include <bits/stdc++.h>
using namespace std;

// Function to return a string
// found from the coded string
string findstring(string str)
{
    // Declaring string to store result
    string result = "";

    // Loop to generate the original string
    for (int i = 0; str[i] != '\0'; i++) {
        // If current character in string
        // is '*' add result to itself
        if (str[i] == '*')
            result += result;

        // Else add current element only
        else
            result += str[i];
    }
    // Return the result
    return result;
}

// Driver code
int main()
{
    string str = "ab*c*d";
    cout << findstring(str);
    return 0;
}

// Java code to implement above approach
class GFG {

    // Function to return a string
    // found from the coded string
    static String findstring(String str)
    {

        // Declaring string to store result
        String result = "";

        // Loop to generate the original string
        for (int i = 0; i < str.length(); i++) {

            // If current character in string
            // is '*' add result to itself
            if (str.charAt(i) == '*')
                result += result;

            // Else add current element only
            else
                result += str.charAt(i);
        }

        // Return the result
        return result;
    }

    // Driver code
    public static void main(String[] args)
    {
        String str = "ab*c*d";
        System.out.println(findstring(str));
    }
}

// This code is contributed by ukasp.

# Python code for the above approach

# Function to return a string
# found from the coded string
def findstring(str):
  
    # Declaring string to store result
    result = ""

    # Loop to generate the original string
    for i in range(len(str)):
      
        # If current character in string
        # is '*' add result to itself
        if (str[i] == '*'):
                result += result

        # Else add current element only
        else:
                result += str[i]
    # Return the result
    return result

# Driver code
str = "ab*c*d"
print(findstring(str))

# This code is contributed by Saurabh Jaiswal

// C# code to implement above approach
using System;
class GFG
{

  // Function to return a string
  // found from the coded string
  static string findstring(string str)
  {

    // Declaring string to store result
    string result = "";

    // Loop to generate the original string
    for (int i = 0; i < str.Length; i++)
    {

      // If current character in string
      // is '*' add result to itself
      if (str[i] == '*')
        result += result;

      // Else add current element only
      else
        result += str[i];
    }

    // Return the result
    return result;
  }

  // Driver code
  public static void Main()
  {
    string str = "ab*c*d";
    Console.Write(findstring(str));

  }
}

// This code is contributed by Samim Hossain Mondal.

 <script>
    // JavaScript code for the above approach


    // Function to return a string
    // found from the coded string
    function findstring(str) {
      // Declaring string to store result
      let result = "";

      // Loop to generate the original string
      for (let i = 0; i < str.length; i++) {
        // If current character in string
        // is '*' add result to itself
        if (str[i] == '*')
          result += result;

        // Else add current element only
        else
          result += str[i];
      }
      // Return the result
      return result;
    }

    // Driver code

    let str = "ab*c*d";
    document.write(findstring(str));

  // This code is contributed by Potta Lokesh
  </script>

ababcababcd

Time Complexity: O(N)
Auxiliary Space: O(N)