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Queries for rotation and Kth character of the given string in constant time
  • Difficulty Level : Easy
  • Last Updated : 13 Sep, 2019

Given a string str, the task is to perform the following type of queries on the given string:

  1. (1, K): Left rotate the string by K characters.
  2. (2, K): Print the Kth character of the string.

Examples:

Input: str = “abcdefgh”, q[][] = {{1, 2}, {2, 2}, {1, 4}, {2, 7}}
Output:
d
e
Query 1: str = “cdefghab”
Query 2: 2nd character is d
Query 3: str = “ghabcdef”
Query 4: 7th character is e

Input: str = “abc”, q[][] = {{1, 2}, {2, 2}}
Output:
a

Approach: The main observation here is that the string doesn’t need to be rotated in every query instead we can create a pointer ptr pointing to the first character of the string and which can be updated for every rotation as ptr = (ptr + K) % N where K the integer by which the string needs to be rotated and N is the length of the string. Now for every query of the second type, the Kth character can be found by str[(ptr + K – 1) % N].



Below is the implementation of the above approach:

C++

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// C++ implementation of the approach
#include <bits/stdc++.h>
using namespace std;
  
#define size 2
  
// Function to perform the required
// queries on the given string
void performQueries(string str, int n,
                    int queries[][size], int q)
{
  
    // Pointer pointing to the current starting
    // character of the string
    int ptr = 0;
  
    // For every query
    for (int i = 0; i < q; i++) {
  
        // If the query is to rotate the string
        if (queries[i][0] == 1) {
  
            // Update the pointer pointing to the
            // starting character of the string
            ptr = (ptr + queries[i][1]) % n;
        }
        else {
  
            int k = queries[i][1];
  
            // Index of the kth character in the
            // current rotation of the string
            int index = (ptr + k - 1) % n;
  
            // Print the kth character
            cout << str[index] << "\n";
        }
    }
}
  
// Driver code
int main()
{
    string str = "abcdefgh";
    int n = str.length();
  
    int queries[][size] = { { 1, 2 }, { 2, 2 }, 
                            { 1, 4 }, { 2, 7 } };
    int q = sizeof(queries) / sizeof(queries[0]);
  
    performQueries(str, n, queries, q);
  
    return 0;
}

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Java

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// Java implementation of the above approach
import java.util.*;
class GFG 
{
static int size = 2;
  
// Function to perform the required
// queries on the given string
static void performQueries(String str, int n,
                           int queries[][], int q)
{
  
    // Pointer pointing to the current 
    // starting character of the string
    int ptr = 0;
  
    // For every query
    for (int i = 0; i < q; i++) 
    {
  
        // If the query is to rotate the string
        if (queries[i][0] == 1)
        {
  
            // Update the pointer pointing to the
            // starting character of the string
            ptr = (ptr + queries[i][1]) % n;
        }
        else 
        {
            int k = queries[i][1];
  
            // Index of the kth character in the
            // current rotation of the string
            int index = (ptr + k - 1) % n;
  
            // Print the kth character
            System.out.println(str.charAt(index));
        }
    }
}
  
// Driver code
public static void main(String[] args)
{
    String str = "abcdefgh";
    int n = str.length();
  
    int queries[][] = { { 1, 2 }, { 2, 2 }, 
                        { 1, 4 }, { 2, 7 } };
    int q = queries.length;
  
    performQueries(str, n, queries, q);
}
  
// This code is contributed by 29AjayKumar

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Python3

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# Python3 implementation of the approach 
size = 2
  
# Function to perform the required 
# queries on the given string 
def performQueries(string, n, queries, q) : 
  
    # Pointer pointing to the current starting 
    # character of the string 
    ptr = 0
  
    # For every query 
    for i in range(q) :
  
        # If the query is to rotate the string 
        if (queries[i][0] == 1) : 
  
            # Update the pointer pointing to the 
            # starting character of the string 
            ptr = (ptr + queries[i][1]) % n; 
              
        else :
  
            k = queries[i][1]; 
  
            # Index of the kth character in the 
            # current rotation of the string 
            index = (ptr + k - 1) % n; 
  
            # Print the kth character 
            print(string[index]); 
  
# Driver code 
if __name__ == "__main__"
  
    string = "abcdefgh"
    n = len(string); 
  
    queries = [[ 1, 2 ], [ 2, 2 ], 
               [ 1, 4 ], [ 2, 7 ]]; 
    q = len(queries); 
  
    performQueries(string, n, queries, q); 
      
# This code is contributed by AnkitRai01

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C#

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// C# implementation of the approach
using System;
      
class GFG 
{
static int size = 2;
  
// Function to perform the required
// queries on the given string
static void performQueries(String str, int n,
                           int [,]queries, int q)
{
  
    // Pointer pointing to the current 
    // starting character of the string
    int ptr = 0;
  
    // For every query
    for (int i = 0; i < q; i++) 
    {
  
        // If the query is to rotate the string
        if (queries[i, 0] == 1)
        {
  
            // Update the pointer pointing to the
            // starting character of the string
            ptr = (ptr + queries[i, 1]) % n;
        }
        else
        {
            int k = queries[i, 1];
  
            // Index of the kth character in the
            // current rotation of the string
            int index = (ptr + k - 1) % n;
  
            // Print the kth character
            Console.WriteLine(str[index]);
        }
    }
}
  
// Driver code
public static void Main(String[] args)
{
    String str = "abcdefgh";
    int n = str.Length;
  
    int [,]queries = { { 1, 2 }, { 2, 2 }, 
                       { 1, 4 }, { 2, 7 } };
    int q = queries.GetLength(0);
  
    performQueries(str, n, queries, q);
}
}
  
// This code is contributed by PrinciRaj1992

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Output:

d
e

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