Length of the largest substring which have character with frequency greater than or equal to half of the substring

Given a string S consisting of character from ‘a’ to ‘z’. The task is to find the length of the largest substring of S which contains a character whose frequency in the sub-string is greater than or equal to half of the length of the substring.

Note: For odd-length substring, to calculate half-length consider integer division. For Example, half of 11 is 5.

Examples:

Input : S = "ababbbacbcbcca" 
Output : 13
Substring from index 1(0-based indexing) to index 13, 
"babbbacbcbcca" have b's frequency equals to 6 which is 
equal to half of the length of substring (13/2 = 6).
 
Input : S = "abcde"
Output : 3

Simple Approach: The idea is to find all the substring of S and for each substring find the frequency of each character and compare it with half of the length of the substring. Now, among all substrings satisfying the condition, output the length of the one having largest length.

Effcient Approach:
First, observe there can be only 26 distinct characters in the string S. So, consider each character one by one for having the frequency greater than or equal to the length of the sub string.

So, to find the length of the longest possible sub string which satisfy the condition, for each character we will make the prefix array of the count of the character. For example, for S = “abacdaef”, prefix array for ‘a’ will be, say freq[], [1, 1, 2, 2, 2, 3, 3, 3].

We are looking for following condition to satisfy:

freq[r] - freq[l - 1] >= (r - l)/2, where l, r are the indices.

Also, we can write,

(2 * freq[r]) - r >= (2 * freq[l - 1]) - l

So, find two arrays r[] and l[], where r[i] = (2 * freq[i]) – i and l[i] = (2 * freq[l – 1]) – l, for 1 <= i <= length of S.

Now, for every i in r[], find the lower bound in l[] using binary search such that the index is minimum in l[], say j.

So, i – j + 1 is one of the solution, find the maximum one.

Then loop the above method for each character.

Below is the implementation of the above approach:

C++

filter_none

edit
close

play_arrow

link
brightness_4
code

// C++ implementation of the above approach
  
#include <bits/stdc++.h>
using namespace std;
  
// Function to return the length of the longest 
// sub string having frequency of a character 
// greater than half of the length of the 
// sub string
int maxLength(string s, int n)
{
    int ans = INT_MIN;
    vector<int> A, L, R;
    int freq[n + 5];
  
    // for each of the character 'a' to 'z'
    for (int i = 0; i < 26; i++) {
        int count = 0;
        memset(freq, 0, sizeof freq);
  
        // finding frequency prefix array of the 
        // character
        for (int j = 0; j < n; j++) {
            if (s[j] - 'a' == i)
                count++;
            freq[j] = count;
        }
  
        // Finding the r[] and l[] arrays.
        for (int j = 0; j < n; j++) {
            L.push_back((2 * freq[j - 1]) - j);
            R.push_back((2 * freq[j]) - j);
        }
  
        int max_len = INT_MIN;
        int min_val = INT_MAX;
  
        // for each j from 0 to n
        for (int j = 0; j < n; j++) {
            min_val = min(min_val, L[j]);
            A.push_back(min_val);
  
            int l = 0, r = j;
  
            // Finding the lower bound of i.
            while (l <= r) {
                int mid = (l + r) >> 1;
                if (A[mid] <= R[j]) {
                    max_len = max(max_len, j - mid + 1);
                    r = mid - 1;
                }
                else {
                    l = mid + 1;
                }
            }
        }
  
        // storing the maximum value of i - j + 1
        ans = max(ans, max_len);
  
        // clearing all the vector so that it clearing
        // be use for other character.
        A.clear();
        R.clear();
        L.clear();
    }
  
    return ans;
}
  
// Driver Code
int main()
{
    string s = "ababbbacbcbcca";
    int n = s.length();
  
    cout << maxLength(s, n) << '\n';
      
    return 0;
}

chevron_right


Python3

filter_none

edit
close

play_arrow

link
brightness_4
code

# Python3 implementation of the above approach 
import sys
  
# Function to return the length 
# of the longest sub string 
# having frequency of a character 
# greater than half of the length 
# of the sub string 
def maxLength(s, n) :
      
    ans = -(sys.maxsize + 1); 
    A, L, R = [], [], []; 
    freq = [0] * (n + 5); 
  
    # for each of the character 'a' to 'z' 
    for i in range(26) :
        count = 0
          
        # finding frequency prefix array 
        # of the character 
        for j in range(n) :
            if (ord(s[j]) - ord('a') == i) :
                count += 1
                  
            freq[j] = count; 
          
        # Finding the r[] and l[] arrays. 
        for j in range(n) : 
            L.append((2 * freq[j - 1]) - j); 
            R.append((2 * freq[j]) - j); 
          
        max_len = -(sys.maxsize + 1); 
        min_val = sys.maxsize ; 
  
        # for each j from 0 to n 
        for j in range(n) :
            min_val = min(min_val, L[j]); 
            A.append(min_val); 
  
            l = 0; r = j; 
  
            # Finding the lower bound of i. 
            while (l <= r) : 
                mid = (l + r) >> 1
                if (A[mid] <= R[j]) :
                    max_len = max(max_len, j - mid + 1); 
                    r = mid - 1
                  
                else :
                    l = mid + 1
  
        # storing the maximum value of i - j + 1 
        ans = max(ans, max_len); 
  
        # clearing all the vector so that it can
        # be used for other characters. 
        A.clear(); 
        R.clear(); 
        L.clear(); 
  
    return ans; 
  
# Driver Code 
if __name__ == "__main__"
  
    s = "ababbbacbcbcca"
    n = len(s); 
  
    print(maxLength(s, n));
  
# This code is contributed by AnkitRai01

chevron_right


Output:

13


My Personal Notes arrow_drop_up

Check out this Author's contributed articles.

If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please Improve this article if you find anything incorrect by clicking on the "Improve Article" button below.



Improved By : AnkitRai01, nidhi_biet