Smallest alphabet greater than a given character

Given a list of sorted characters consisting of both Uppercase and Lowercase Alphabets and a particular target value, say K, the task is to find the smallest element in the list that is larger than K.
Letters also wrap around. For example, if K = ‘z’ and letters = [‘A’, ‘r’, ‘z’], then the answer would be ‘A’.

Examples:

Input : Letters = ["D", "J", "K"]
        K = "B"
Output: 'D'
Explanation:
The Next greater character of "B" is 'D'
since it is the smallest element from the 
set of given letters, greater than "B".

Input:  Letters = ["h", "n", "s"]
        K = "t"
Output: 'h'

Prerequisites: Binary Search

Approach: Binary Search can be applied to find the index of the smallest character in the given Set of Letters such that the character at that index is greater than K. If the element at the current mid is smaller than or equal to K, binary search is applied on the Right half, else it is applied on the left half.

CPP

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/* C++ Program to find the smallest character 
   from the given set of letter, which is greater
   than the target element */
#include <bits/stdc++.h>
using namespace std;
  
/* Returns the smallest character from the given 
   set of letters that is greater than K */
char nextGreatestAlphabet(vector<char>& alphabets, 
                                          char K)
{
  
    int l = 0, r = alphabets.size() - 1;
  
    // Take the first element as l and
    // the rightmost element as r
    while (l < r && alphabets[r] > K) {
  
        // if this while condition does not satisfy
        // simply return the first element.
        int mid = (l + r) / 2;
        if (alphabets[mid] > K)
            r = mid;
        else
            l = mid + 1;
    }
  
    // Return the smallest element
    return alphabets[l];
}
  
// Driver Code
int main()
{
    vector<char> letters{ 'A', 'K', 'S' };
    char K = 'L';
    char result = nextGreatestAlphabet(letters, K);
    cout << result << endl;
    return 0;
}

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Java

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/* Java Program to find the smallest character 
from the given set of letter, which is greater
than the target element */
  
class GFG { 
      
    /* Returns the smallest character from the given 
    set of letters that is greater than K */
    static char nextGreatestAlphabet(char alphabets[], 
                                              char K)
    {
      
        int l = 0, r = alphabets.length - 1;
      
        // Take the first element as l and
        // the rightmost element as r
        while (l < r && alphabets[r] > K)
        {
      
            // if this while condition does not
            // satisfy simply return the first
            // element.
            int mid = (l + r) / 2;
            if (alphabets[mid] > K)
                r = mid;
            else
                l = mid + 1;
        }
      
        // Return the smallest element
        return alphabets[l];
    }
      
    // Driver Code
    public static void main(String [] args)
    {
        char letters[] = { 'A', 'K', 'S' };
        char K = 'L';
        char result = 
              nextGreatestAlphabet(letters, K);
                
        System.out.println(result);
    }
}
  
// This code is contributed by Smitha.

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Python 3

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# Python 3 Program to find the smallest
# character from the given set of letter,
# which is greater than the target 
# element */
  
# Returns the smallest character from 
# the given set of letters that is 
# greater than K 
def nextGreatestAlphabet(alphabets, K):
  
    l = 0
    r = len(alphabets) - 1
  
    # Take the first element as l and
    # the rightmost element as r
    while (l < r and alphabets[r] > K) :
  
        # if this while condition does
        # not satisfy simply return the
        # first element.
        mid = int((l + r) / 2)
          
        if (alphabets[mid] > K):
            r = mid
        else:
            l = mid + 1
      
    # Return the smallest element
    return alphabets[l]
  
# Driver Code
letters = ['A', 'K', 'S']
K = 'L'
result = nextGreatestAlphabet(letters, K)
print(result)
  
# This code is contributed by Smitha

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C#

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/* C# Program to find the smallest character 
from the given set of letter, which is greater
than the target element */
using System;
  
class GFG {
      
    /* Returns the smallest character from the given 
    set of letters that is greater than K */
    static char nextGreatestAlphabet(char []alphabets, 
                                              char K)
    {
      
        int l = 0, r = alphabets.Length - 1;
      
        // Take the first element as l and
        // the rightmost element as r
        while (l < r && alphabets[r] > K)
        {
      
            // if this while condition does not
            // satisfy simply return the first
            // element.
            int mid = (l + r) / 2;
              
            if (alphabets[mid] > K)
                r = mid;
            else
                l = mid + 1;
        }
      
        // Return the smallest element
        return alphabets[l];
    }
      
    // Driver Code
    public static void Main()
    {
        char []letters = { 'A', 'K', 'S' };
        char K = 'L';
        char result =
             nextGreatestAlphabet(letters, K);
               
        Console.Write(result);
    }
}
  
// This code is contributed by Smitha

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Output:

S

The Time Complexity of the above approach is, O(log N) where N is the number of characters in the given set of Letters.



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