Queries to find frequencies of a string within specified substrings
Last Updated :
26 May, 2021
Given a string S and a matrix Q of queries, each specifying the starting and ending indices L( = Q[i][0]) and R( = Q[i][0]) respectively of a substring of S, the task is to find the frequency of string K in substring [L, R].
Note: The ranges follow the 1-based indexing.
Examples:
Input: S = "GFGFFGFG", K = "GFG", Q = {{1, 8}, {3, 5}, {5, 8}}
Output:
2
0
1
Explanation: For query 1, there are 2 ("GFG") substrings from index 1 to index 8. One is from index 1 to 3 and the other is from index 6 to 8.
For query 2, there are 0 ("GFG") substrings from index 3 to 5.
For query 3, there are 1 ("GFG") substrings from index 5 to index 8. The one and only substring are from index 6 to 8.
Input: S = "ABCABCABABC", K = "ABC", Q = {{1, 6}, {5, 11}}
Output:
2
1
Naive Approach:
Run a loop from L to R for all the queries. Count occurrence of the string K and return count.
Time Complexity: O(length of Q * |S|).
Efficient Approach:
Pre-compute and store the frequency of K for every index. Now, for computing the frequency of the string in a range [L, R], we just need to calculate the difference between the frequency of K at index (R-1) and (L-1).
Below is the implementation of the above approach:
C++
// C++ Program to find
// frequency of a string K
// in a substring [L, R] in S
#include <bits/stdc++.h>
#define max_len 100005
using namespace std;
// Store the frequency of
// string for each index
int cnt[max_len];
// Compute and store frequencies
// for every index
void precompute(string s, string K)
{
int n = s.size();
for (int i = 0; i < n - 1; i++) {
cnt[i + 1]
= cnt[i]
+ (s.substr(i, K.size()) == K);
}
}
// Driver Code
int main()
{
string s = "ABCABCABABC";
string K = "ABC";
precompute(s, K);
vector<pair<int, int> > Q
= { { 1, 6 }, { 5, 11 } };
for (auto it : Q) {
cout << cnt[it.second - 1]
- cnt[it.first - 1]
<< endl;
}
return 0;
}
Java
// Java program to find
// frequency of a string K
// in a substring [L, R] in S
class GFG{
static int max_len = 100005;
// Store the frequency of
// string for each index
static int cnt[] = new int[max_len];
// Compute and store frequencies
// for every index
public static void precompute(String s,
String K)
{
int n = s.length();
for(int i = 0; i < n - 2; i++)
{
cnt[i + 1] = cnt[i];
if (s.substring(
i, i + K.length()).equals(K))
{
cnt[i + 1] += 1;
}
}
cnt[n - 2 + 1] = cnt[n - 2];
}
// Driver code
public static void main(String[] args)
{
String s = "ABCABCABABC";
String K = "ABC";
precompute(s, K);
int Q[][] = { { 1, 6 }, { 5, 11 } };
for(int it = 0; it < Q.length; it++)
{
System.out.println(cnt[Q[it][1] - 1] -
cnt[Q[it][0] - 1]);
}
}
}
// This code is contributed by divyesh072019
Python3
# Python3 Program to find
# frequency of a string K
# in a substring [L, R] in S
max_len = 100005
# Store the frequency of
# string for each index
cnt = [0] * max_len
# Compute and store frequencies
# for every index
def precompute(s, K):
n = len(s)
for i in range(n - 1):
cnt[i + 1] = cnt[i]
if s[i : len(K) + i] == K:
cnt[i + 1] += 1
# Driver Code
if __name__ == "__main__":
s = "ABCABCABABC"
K = "ABC"
precompute(s, K)
Q = [[1, 6], [5, 11]]
for it in Q:
print(cnt[it[1] - 1] -
cnt[it[0] - 1])
# This code is contributed by Chitranayal
C#
// C# program to find frequency of
// a string K in a substring [L, R] in S
using System.IO;
using System;
class GFG{
static int max_len = 100005;
// Store the frequency of
// string for each index
static int[] cnt = new int[max_len];
// Compute and store frequencies
// for every index
static void precompute(string s,string K)
{
int n = s.Length;
for(int i = 0; i < n - 2; i++)
{
cnt[i + 1] = cnt[i];
if (s.Substring(i, K.Length).Equals(K))
{
cnt[i + 1] += 1;
}
}
cnt[n - 2 + 1] = cnt[n - 2];
}
// Driver code
static void Main()
{
string s = "ABCABCABABC";
string K = "ABC";
precompute(s, K);
int[,] Q = { { 1, 6 }, { 5, 11 } };
for(int it = 0; it < Q.GetLength(0); it++)
{
Console.WriteLine(cnt[Q[it, 1] - 1] -
cnt[Q[it, 0] - 1]);
}
}
}
// This code is contributed by rag2127
JavaScript
<script>
// Javascript program to find
// frequency of a string K
// in a substring [L, R] in S
var max_len = 100005;
// Store the frequency of
// string for each index
var cnt = Array(max_len).fill(0);
// Compute and store frequencies
// for every index
function precompute(s, K)
{
var n = s.length;
for(var i = 0; i < n - 1; i++)
{
cnt[i + 1] = cnt[i] +
(s.substring(i, i + K.length) == K);
}
}
// Driver Code
var s = "ABCABCABABC";
var K = "ABC";
precompute(s, K);
var Q = [ [ 1, 6 ], [ 5, 11 ] ];
Q.forEach((it) => {
document.write(cnt[it[1] - 1] -
cnt[it[0] - 1] + "<br>");
});
// This code is contributed by itsok
</script>
Time Complexity: O( | S | + length of Q ), as every query is answered in O(1).
Auxiliary Space: O( |S| )
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