Kth most frequent Character in a given String

Given a string str and an integer K, the task is to find the K-th most frequent character in the string. If there are multiple characters that can account as K-th most frequent character then, print any one of them.

Examples:

Input: str = “GeeksforGeeks”, K = 3
Output: f
Explanation:
K = 3, here ‘e’ appears 4 times
& ‘g’, ‘k’, ‘s’ appears 2 times
& ‘o’, ‘f’, ‘r’ appears 1 time.
Any output from ‘o’ (or) ‘f’ (or) ‘r’ will be correct.

Input: str = “trichotillomania”, K = 2
Output: l

Approach



Below is the implementation of the above approach.

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// C++ program to find kth most frequent
// character in a string
#include <bits/stdc++.h>
using namespace std;
  
// Used for sorting by frequency.
bool sortByVal(const pair<char, int>& a,
               const pair<char, int>& b)
{
    return a.second > b.second;
}
  
// function to sort elements by frequency
char sortByFreq(string str, int k)
{
    // Store frequencies of characters
    unordered_map<char, int> m;
    for (int i = 0; i < str.length(); ++i)
        m[str[i]]++;
  
    // Copy map to vector
    vector<pair<char, int> > v;
    copy(m.begin(), m.end(), back_inserter(v));
  
    // Sort the element of array by frequency
    sort(v.begin(), v.end(), sortByVal);
  
    // Find k-th most frequent item. Please note
    // that we need to consider only distinct
    int count = 0;
    for (int i = 0; i < v.size(); i++) {
  
        // Increment count only if frequency is
        // not same as previous
        if (i == 0 || v[i].second != v[i - 1].second)
            count++;
  
        if (count == k)
            return v[i].first;
    }
  
    return -1;
}
  
// Driver program
int main()
{
    string str = "geeksforgeeks";
    int k = 3;
    cout << sortByFreq(str, k);
    return 0;
}
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Output:
r

Time Complexity: O(NlogN) Please note that this is an upper bound on time complexity. If we consider alphabet size as constant (for example lower case English alphabet size is 26), we can say time complexity as O(N). The vector size would never be more that alphabet size.
Auxiliary Space: O(N)





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