C++ Program for Queries for rotation and Kth character of the given string in constant time
Given a string str, the task is to perform the following type of queries on the given string:
- (1, K): Left rotate the string by K characters.
- (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++ 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] << " ";
}
}
}
// 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;
}
// C++ implementation of the approachusing namespace std;// Function to perform the required// queries on the given stringvoid 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] << " "; } }}// Driver codeint 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;}Output
d e
Time Complexity: O(Q), Where Q is the number of queries.
Auxiliary Space: O(1)
Please refer complete article on Queries for rotation and Kth character of the given string in constant time for more details!
