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Check if a K-length substring exists having only 2 distinct characters, each with frequency greater than K/3

  • Last Updated : 13 Dec, 2021

Given a string S of lowercase alphabets and an integer K, the task is to find whether there exists a substring of length K having only 2 unique characters and the count of both of the characters must be greater than K/3. If such a string exists, print ‘YES‘ else ‘NO‘.

Examples:

Input: “abbad”, K = 4
Output: YES
Explaination: A substring “abba” has 2 a’s and 2 b’s with total length 4, and since, 2 is greater than 4/3, it is a valid substring.

Input: “abaaaa”, K = 6
Output: NO
Explaination: No valid substring exists.

 

Approach: The task can be solved by checking all the K length substrings using the sliding window technique by keeping track of the number of unique characters in the current substring. Follow the below steps to solve the problem:

  • Take an unordered map to store the number of characters with their frequency in the current substring, and If
    • There are only 2 distinct characters in the current substring, and
    • None of the characters has a count less than or equal to K/3
  • Current substring is a valid string.
  • Hence print YES,
  • If no valid substring has been encountered, print NO.

Below is the implementation of the above approach:

C++




// C++ program for the above approach
#include <bits/stdc++.h>
using namespace std;
 
// Function to check whether
// a valid substring exists or not
void checkGoldenString(string S, int k)
{
    // Store the count of distinct chars
    // with their frequencies
    unordered_map<char, int> occ;
 
    int n = S.length();
 
    // First substring of length k
    for (int i = 0; i < k; i++)
        occ[S[i]]++;
 
    // Check if it is valid
    if (occ.size() == 2) {
        int x = -1, y = -1;
        for (auto item : occ) {
            if (x == -1)
                x = item.second;
            else
                y = item.second;
        }
 
        // Count of one of the chars is
        // greater than k/3
        if (x >= k / 3 && y >= k / 3) {
            cout << "YES\n";
            return;
        }
    }
 
    // Sliding over the entire string
    for (int i = k; i < n; i++) {
 
        // Discarding first character of
        // previous window
        occ[S[i - k]]--;
 
        // Erase it from the map, if its
        // frequency becomes 0
        if (occ[S[i - k]] == 0)
            occ.erase(S[i - k]);
 
        // Increment count of current char
        occ[S[i]]++;
 
        // Checking valid or not
        if (occ.size() == 2) {
            int x = -1, y = -1;
            for (auto item : occ) {
                if (x == -1)
                    x = item.second;
                else
                    y = item.second;
            }
 
            if (x >= k / 3 && y >= k / 3) {
                cout << "YES\n";
                return;
            }
        }
    }
 
    // No valid substring is found
    cout << "NO\n";
}
 
// Driver Code
int main()
{
    int K = 6;
    string S = "abaaaa";
    checkGoldenString(S, K);
 
    K = 4;
    S = "abbaaaa";
    checkGoldenString(S, K);
}

Java




// Java program for the above approach
import java.util.HashMap;
class GFG {
 
    // Function to check whether
    // a valid substring exists or not
    public static void checkGoldenString(String S, int k)
    {
       
        // Store the count of distinct chars
        // with their frequencies
        HashMap<Character, Integer> occ = new HashMap<Character, Integer>();
 
        int n = S.length();
 
        // First substring of length k
        for (int i = 0; i < k; i++) {
            if (occ.containsKey(S.charAt(i))) {
                occ.put(S.charAt(i), occ.get(S.charAt(i)) + 1);
            } else {
                occ.put(S.charAt(i), 1);
            }
        }
 
        // Check if it is valid
        if (occ.size() == 2) {
            int x = -1, y = -1;
            for (char item : occ.keySet()) {
                if (x == -1)
                    x = occ.get(item);
                else
                    y = occ.get(item);
            }
 
            // Count of one of the chars is
            // greater than k/3
            if (x >= k / 3 && y >= k / 3) {
                System.out.println("YES");
                return;
            }
        }
 
        // Sliding over the entire string
        for (int i = k; i < n; i++) {
 
            // Discarding first character of
            // previous window
            occ.put(S.charAt(i - k), occ.get(S.charAt(i - k)) - 1);
 
            // Erase it from the map, if its
            // frequency becomes 0
            if (occ.get(S.charAt(i - k)) == 0)
                occ.remove(S.charAt(i - k));
 
            // Increment count of current char
            if (occ.containsKey(S.charAt(i))) {
                occ.put(S.charAt(i), occ.get(S.charAt(i)) + 1);
            } else {
                occ.put(S.charAt(i), 1);
            }
 
            // Checking valid or not
            if (occ.size() == 2) {
                int x = -1, y = -1;
                for (char item : occ.keySet()) {
                    if (x == -1)
                        x = occ.get(item);
                    else
                        y = occ.get(item);
                }
 
                if (x >= k / 3 && y >= k / 3) {
                    System.out.println("YES");
                    return;
                }
            }
        }
 
        // No valid substring is found
        System.out.println("NO");
    }
 
    // Driver Code
    public static void main(String args[]) {
        int K = 6;
        String S = "abaaaa";
        checkGoldenString(S, K);
 
        K = 4;
        S = "abbaaaa";
        checkGoldenString(S, K);
    }
}
 
// This code is contributed by saurabh_jaiswal.

Python3




# Python Program to implement
# the above approach
 
# Function to check whether
# a valid substring exists or not
def checkGoldenString(S, k):
 
    # Store the count of distinct chars
    # with their frequencies
    occ = dict()
 
    n = len(S)
 
    # First substring of length k
    for i in range(k):
        if (S[i] not in occ):
            occ[S[i]] = 1
        else:
            occ[S[i]] += 1
 
    # Check if it is valid
    if (len(occ) == 2):
        x = -1
        y = -1
        for z in occ.values():
            if (x == -1):
                x = z
            else:
                y = z
 
        # Count of one of the chars is
        # greater than k/3
        if (x >= k // 3 and y >= k // 3):
            print("YES")
            return
 
    # Sliding over the entire string
    for i in range(k, n):
 
        # Discarding first character of
        # previous window
        if (S[i - k] in occ):
            occ[[S[i - k]]] -= k
 
        # Erase it from the map, if its
        # frequency becomes 0
        if (occ[S[i - k]] == 0):
            del occ[S[i - k]]
 
        # Increment count of current char
        if (S[i] not in occ):
            occ[S[i]] = 1
        else:
            occ[S[i]] += 1
 
        # Checking valid or not
        if (len(occ) == 2):
            x = -1
            y = -1
            for z in occ.values:
                if (x == -1):
                    x = z
                else:
                    y = z
 
            if (x >= k // 3 and y >= k // 3):
                print("YES" + '<br>')
                return
 
    # No valid substring is found
    print("NO")
 
# Driver Code
K = 6
S = "abaaaa"
checkGoldenString(S, K)
 
K = 4
S = "abbaaaa"
checkGoldenString(S, K)
 
# This code is contributed by Saurabh Jaiswal

C#




// C# program for the above approach
using System;
using System.Collections.Generic;
 
public class GFG {
 
    // Function to check whether
    // a valid substring exists or not
    public static void checkGoldenString(String S, int k)
    {
       
        // Store the count of distinct chars
        // with their frequencies
        Dictionary<char, int> occ = new Dictionary<char, int>();
 
        int n = S.Length;
 
        // First substring of length k
        for (int i = 0; i < k; i++) {
            if (occ.ContainsKey(S[i])) {
                occ[S[i]] = occ[S[i]] + 1;
            } else {
                occ.Add(S[i], 1);
            }
        }
 
        // Check if it is valid
        if (occ.Count == 2) {
            int x = -1, y = -1;
            foreach (char item in occ.Keys) {
                if (x == -1)
                    x = occ[item];
                else
                    y = occ[item];
            }
 
            // Count of one of the chars is
            // greater than k/3
            if (x >= k / 3 && y >= k / 3) {
                Console.WriteLine("YES");
                return;
            }
        }
 
        // Sliding over the entire string
        for (int i = k; i < n; i++) {
 
            // Discarding first character of
            // previous window
            occ[S[i - k] ]= occ[S[i - k]] - 1;
 
            // Erase it from the map, if its
            // frequency becomes 0
            if (occ[S[i - k]] == 0)
                occ.Remove(S[i - k]);
 
            // Increment count of current char
            if (occ.ContainsKey(S[i])) {
                occ[S[i]] = occ[S[i]] + 1;
            } else {
                occ.Add(S[i], 1);
            }
 
            // Checking valid or not
            if (occ.Count == 2) {
                int x = -1, y = -1;
                foreach (char item in occ.Keys) {
                    if (x == -1)
                        x = occ[item];
                    else
                        y = occ[item];
                }
 
                if (x >= k / 3 && y >= k / 3) {
                    Console.WriteLine("YES");
                    return;
                }
            }
        }
 
        // No valid substring is found
        Console.WriteLine("NO");
    }
 
    // Driver Code
    public static void Main(String []args) {
        int K = 6;
        String S = "abaaaa";
        checkGoldenString(S, K);
 
        K = 4;
        S = "abbaaaa";
        checkGoldenString(S, K);
    }
}
 
// This code is contributed by shikhasingrajput

Javascript




<script>
 
       // JavaScript Program to implement
       // the above approach
 
       // Function to check whether
       // a valid substring exists or not
       function checkGoldenString(S, k)
       {
        
           // Store the count of distinct chars
           // with their frequencies
           let occ = new Map();
 
           let n = S.length;
 
           // First substring of length k
           for (let i = 0; i < k; i++) {
               if (!occ.has(S[i])) {
                   occ.set(S[i], 1);
               }
               else {
                   occ.set(S[i], occ.get(S[i]) + 1);
               }
           }
 
 
           // Check if it is valid
           if (occ.size == 2) {
               let x = -1, y = -1;
               for (let [key, value] of occ) {
                   if (x == -1)
                       x = value;
                   else
                       y = value;
               }
 
               // Count of one of the chars is
               // greater than k/3
               if (x >= Math.floor(k / 3) && y >= Math.floor(k / 3)) {
                   document.write("YES" + '<br>');
                   return;
               }
           }
 
           // Sliding over the entire string
           for (let i = k; i < n; i++) {
 
               // Discarding first character of
               // previous window
               if (occ.has(S[i - k]))
                   occ.set([S[i - k]], occ.get(S[i - k]) - 1);
 
               // Erase it from the map, if its
               // frequency becomes 0
               if (occ.get(S[i - k]) == 0)
                   occ.delete(S[i - k]);
 
               // Increment count of current char
               if (!occ.has(S[i]))
                   occ.set(S[i], 1);
               else
                   occ.set(S[i], occ.get(S[i]) + 1)
 
               // Checking valid or not
               if (occ.size == 2) {
                   let x = -1, y = -1;
                   for (let [key, value] of occ) {
                       if (x == -1)
                           x = value;
                       else
                           y = value;
                   }
 
                   if (x >= Math.floor(k / 3) && y >= Math.floor(k / 3)) {
                       document.write("YES" + '<br>');
                       return;
                   }
               }
           }
 
           // No valid substring is found
           document.write("NO" + '<br>');
       }
 
       // Driver Code
       let K = 6;
       let S = "abaaaa";
       checkGoldenString(S, K);
 
       K = 4;
       S = "abbaaaa";
       checkGoldenString(S, K);
 
   // This code is contributed by Potta Lokesh
   </script>

 
 

Output
NO
YES

 

Time Complexity: O(N)
Auxiliary Space: O(1)

 


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