# Lexicographically smallest K-length substring containing maximum number of vowels

Given a string str containing only lowercase English alphabets and an integer K, the task is to find a K length substring which contains the maximum number of vowels (i.e. ‘a’, ‘e’, ‘i’, ‘o’, ‘u’). If there are multiple such substrings, return the substring which is lexicographically smallest.

Examples:

Input: str = “geeksforgeeks”, K = 4
Output: eeks
Explanation:
The substrings with maximum count of vowels are “geek”, “eeks” which contains 2 vowels. But “eeks” is lexicographically smallest.

Input: str = “ceebbaceeffo”, K = 3
Output: ace
Explanation:
Lexicographically substrings with maximum count of vowels are “ace”.

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

Naive Approach:
To solve the problem mentioned above we have to generate all the substrings of length K and store the lexicographically smallest of all such substrings which contains the maximum number of vowels.
Time Complexity: O(N2)

Efficient Approach:
The above mentioned procedure can be optimized by creating a prefix sum array pref[] of vowels where the ith index contains the count of vowels from 0 to the ith index. The count of vowels for any substring str[l : r] can be given by pref[r]-pref[l-1]. Then, find the lexicographically smallest substring with the maximum count of vowels.

Below is the implementation of the above approach:

## C++

 `// C++ program to find ` `// lexicographically smallest ` `// K-length substring containing ` `// maximum number of vowels ` ` `  `#include ` `using` `namespace` `std; ` ` `  `// Function that prints the ` `// lexicographically smallest ` `// K-length substring containing ` `// maximum number of vowels ` `string maxVowelSubString( ` `    ``string str, ``int` `K) ` `{ ` `    ``// Store the length of the string ` `    ``int` `N = str.length(); ` ` `  `    ``// Initialize a prefix sum array ` `    ``int` `pref[N]; ` ` `  `    ``// Loop through the string to ` `    ``// create the prefix sum array ` `    ``for` `(``int` `i = 0; i < N; i++) { ` ` `  `        ``// Store 1 at the index ` `        ``// if it is a vowel ` `        ``if` `(str[i] == ``'a'` `            ``or str[i] == ``'e'` `            ``or str[i] == ``'i'` `            ``or str[i] == ``'o'` `            ``or str[i] == ``'u'``) ` `            ``pref[i] = 1; ` ` `  `        ``// Otherwise, store 0 ` `        ``else` `            ``pref[i] = 0; ` ` `  `        ``// Process the prefix array ` `        ``if` `(i) ` `            ``pref[i] += pref[i - 1]; ` `    ``} ` ` `  `    ``// Initialize the variable to store ` `    ``// maximum count of vowels ` `    ``int` `maxCount = pref[K - 1]; ` ` `  `    ``// Initialize the variable ` `    ``// to store substring ` `    ``// with maximum count of vowels ` `    ``string res = str.substr(0, K); ` ` `  `    ``// Loop through the prefix array ` `    ``for` `(``int` `i = K; i < N; i++) { ` ` `  `        ``// Store the current ` `        ``// count of vowels ` `        ``int` `currCount ` `            ``= pref[i] ` `              ``- pref[i - K]; ` ` `  `        ``// Update the result if current count ` `        ``// is greater than maximum count ` `        ``if` `(currCount > maxCount) { ` ` `  `            ``maxCount = currCount; ` `            ``res = str.substr(i - K + 1, K); ` `        ``} ` ` `  `        ``// Update lexicographically smallest ` `        ``// substring if the current count ` `        ``// is equal to the maximum count ` `        ``else` `if` `(currCount == maxCount) { ` ` `  `            ``string temp ` `                ``= str.substr( ` `                    ``i - K + 1, K); ` ` `  `            ``if` `(temp < res) ` `                ``res = temp; ` `        ``} ` `    ``} ` ` `  `    ``// Return the result ` `    ``return` `res; ` `} ` ` `  `// Driver Program ` `int` `main() ` `{ ` `    ``string str = ``"ceebbaceeffo"``; ` `    ``int` `K = 3; ` ` `  `    ``cout << maxVowelSubString(str, K); ` ` `  `    ``return` `0; ` `} `

## Python3

 `# Python3 program to find ` `# lexicographically smallest ` `# K-length substring containing ` `# maximum number of vowels ` ` `  `# Function that prints the ` `# lexicographically smallest ` `# K-length substring containing ` `# maximum number of vowels ` `def` `maxVowelSubString(str1, K): ` `     `  `    ``# Store the length of the string ` `    ``N ``=` `len``(str1) ` ` `  `    ``# Initialize a prefix sum array ` `    ``pref ``=` `[``0` `for` `i ``in` `range``(N)] ` ` `  `    ``# Loop through the string to ` `    ``# create the prefix sum array ` `    ``for` `i ``in` `range``(N): ` `         `  `        ``# Store 1 at the index ` `        ``# if it is a vowel ` `        ``if` `(str1[i] ``=``=` `'a'` `or`  `            ``str1[i] ``=``=` `'e'` `or`  `            ``str1[i] ``=``=` `'i'` `or` `            ``str1[i] ``=``=` `'o'` `or`  `            ``str1[i] ``=``=` `'u'``): ` `            ``pref[i] ``=` `1` ` `  `        ``# Otherwise, store 0 ` `        ``else``: ` `            ``pref[i] ``=` `0` ` `  `        ``# Process the prefix array ` `        ``if` `(i): ` `            ``pref[i] ``+``=` `pref[i ``-` `1``] ` ` `  `    ``# Initialize the variable to  ` `    ``# store maximum count of vowels ` `    ``maxCount ``=` `pref[K ``-` `1``] ` ` `  `    ``# Initialize the variable ` `    ``# to store substring with  ` `    ``# maximum count of vowels ` `    ``res ``=` `str1[``0``:K] ` ` `  `    ``# Loop through the prefix array ` `    ``for` `i ``in` `range``(K, N): ` `         `  `        ``# Store the current ` `        ``# count of vowels ` `        ``currCount ``=` `pref[i] ``-` `pref[i ``-` `K] ` ` `  `        ``# Update the result if current count ` `        ``# is greater than maximum count ` `        ``if` `(currCount > maxCount): ` `            ``maxCount ``=` `currCount ` `            ``res ``=` `str1[i ``-` `K ``+` `1` `: i ``+` `1``] ` ` `  `        ``# Update lexicographically smallest ` `        ``# substring if the current count ` `        ``# is equal to the maximum count ` `        ``elif` `(currCount ``=``=` `maxCount): ` `            ``temp ``=` `str1[i ``-` `K ``+` `1` `: i ``+` `1``] ` ` `  `            ``if` `(temp < res): ` `                ``res ``=` `temp ` ` `  `    ``# Return the result ` `    ``return` `res ` ` `  `# Driver code ` `if` `__name__ ``=``=` `'__main__'``: ` `     `  `    ``str1 ``=` `"ceebbaceeffo"` `    ``K ``=` `3` ` `  `    ``print``(maxVowelSubString(str1, K)) ` ` `  `# This code is contributed by Surendra_Gangwar `

## C#

 `// C# program to find ` `// lexicographically smallest ` `// K-length substring containing ` `// maximum number of vowels ` `using` `System; ` `class` `GFG{ ` ` `  `// Function that prints the ` `// lexicographically smallest ` `// K-length substring containing ` `// maximum number of vowels ` `static` `string` `maxVowelSubString(``string` `str, ` `                                ``int` `K) ` `{ ` `    ``// Store the length of the string ` `    ``int` `N = str.Length; ` ` `  `    ``// Initialize a prefix sum array ` `    ``int` `[]pref = ``new` `int``[N]; ` ` `  `    ``// Loop through the string to ` `    ``// create the prefix sum array ` `    ``for` `(``int` `i = 0; i < N; i++)  ` `    ``{ ` ` `  `        ``// Store 1 at the index ` `        ``// if it is a vowel ` `        ``if` `(str[i] == ``'a'` `|| ` `            ``str[i] == ``'e'` `||  ` `            ``str[i] == ``'i'` `||  ` `            ``str[i] == ``'o'` `||  ` `            ``str[i] == ``'u'``) ` `            ``pref[i] = 1; ` ` `  `        ``// Otherwise, store 0 ` `        ``else` `            ``pref[i] = 0; ` ` `  `        ``// Process the prefix array ` `        ``if` `(i != 0) ` `            ``pref[i] += pref[i - 1]; ` `    ``} ` ` `  `    ``// Initialize the variable to store ` `    ``// maximum count of vowels ` `    ``int` `maxCount = pref[K - 1]; ` ` `  `    ``// Initialize the variable ` `    ``// to store substring ` `    ``// with maximum count of vowels ` `    ``string` `res = str.Substring(0, K); ` ` `  `    ``// Loop through the prefix array ` `    ``for` `(``int` `i = K; i < N; i++) ` `    ``{ ` ` `  `        ``// Store the current ` `        ``// count of vowels ` `        ``int` `currCount = pref[i] -  ` `                        ``pref[i - K]; ` ` `  `        ``// Update the result if current count ` `        ``// is greater than maximum count ` `        ``if` `(currCount > maxCount)  ` `        ``{ ` `            ``maxCount = currCount; ` `            ``res = str.Substring(i - K + 1, K); ` `        ``} ` ` `  `        ``// Update lexicographically smallest ` `        ``// substring if the current count ` `        ``// is equal to the maximum count ` `        ``else` `if` `(currCount == maxCount) ` `        ``{ ` `            ``string` `temp = str.Substring(i - K + 1, K); ` ` `  `            ``if` `(``string``.Compare(temp, res) == -1) ` `                ``res = temp; ` `        ``} ` `    ``} ` ` `  `    ``// Return the result ` `    ``return` `res; ` `} ` ` `  `// Driver Code ` `public` `static` `void` `Main() ` `{ ` `    ``string` `str = ``"ceebbaceeffo"``; ` `    ``int` `K = 3; ` ` `  `    ``Console.Write(maxVowelSubString(str, K)); ` `} ` `} ` ` `  `// This code is contributed by Code_Mech `

Output:

```ace
```

Time Complexity: O(N) My Personal Notes arrow_drop_up Check out this Author's contributed articles.

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