Minimum length substring with each letter occurring both in uppercase and lowercase

// Java program for the above approach
import java.io.*;
import java.util.*;

class GFG{

// Function to check if the current
// string is balanced or not
static boolean balanced(int small[], 
                        int caps[])
{
    
    // For every character, check if
    // there exists uppercase as well
    // as lowercase characters
    for(int i = 0; i < 26; i++) 
    {
        if (small[i] != 0 && (caps[i] == 0))
            return false;

        else if ((small[i] == 0) && (caps[i] != 0))
            return false;
    }
    return true;
}

// Function to find smallest length substring
// in the given string which is balanced
static void smallestBalancedSubstring(String s)
{
    
    // Store frequency of
    // lowercase characters
    int[] small = new int[26];

    // Stores frequency of
    // uppercase characters
    int[] caps = new int[26];

    Arrays.fill(small, 0);
    Arrays.fill(caps, 0);

    // Count frequency of characters
    for(int i = 0; i < s.length(); i++)
    {
        if (s.charAt(i) >= 65 && s.charAt(i) <= 90)
            caps[s.charAt(i) - 'A']++;
        else
            small[s.charAt(i) - 'a']++;
    }

    // Mark those characters which
    // are not present in both
    // lowercase and uppercase
    Map<Character, 
        Integer> mp = new HashMap<Character, 
                                  Integer>();

    for(int i = 0; i < 26; i++) 
    {
        if (small[i] != 0 && caps[i] == 0)
            mp.put((char)(i + 'a'), 1);
        else if (caps[i] != 0 && small[i] == 0)
            mp.put((char)(i + 'A'), 1);
        // mp[char(i + 'A')] = 1;
    }

    // Initialize the frequencies
    // back to 0
    Arrays.fill(small, 0);
    Arrays.fill(caps, 0);

    // Marks the start and
    // end of current substring
    int i = 0, st = 0;

    // Marks the start and end
    // of required substring
    int start = -1, end = -1;

    // Stores the length of
    // smallest balanced substring
    int minm = Integer.MAX_VALUE;

    while (i < s.length()) 
    {
        if (mp.get(s.charAt(i)) != null) 
        {
            
            // Remove all characters
            // obtained so far
            while (st < i)
            {
                if (s.charAt(st) >= 65 && 
                    s.charAt(st) <= 90)
                    caps[s.charAt(st) - 'A']--;
                else
                    small[s.charAt(st) - 'a']--;

                st++;
            }
            i += 1;
            st = i;
        }
        else 
        {
            if (s.charAt(i) >= 65 && s.charAt(i) <= 90)
                caps[s.charAt(i) - 'A']++;
            else
                small[s.charAt(i) - 'a']++;

            // Remove extra characters from
            // front of the current substring
            while (true)
            {
                if (s.charAt(st) >= 65 && 
                    s.charAt(st) <= 90 && 
                    caps[s.charAt(st) - 'A'] > 1) 
                {
                    caps[s.charAt(st) - 'A']--;
                    st++;
                }
                else if (s.charAt(st) >= 97 && 
                         s.charAt(st) <= 122 && 
                         small[s.charAt(st) - 'a'] > 1)
                {
                    small[s.charAt(st) - 'a']--;
                    st++;
                }
                else
                    break;
            }

            // If substring (st, i) is balanced
            if (balanced(small, caps)) 
            {
                if (minm > (i - st + 1)) 
                {
                    minm = i - st + 1;
                    start = st;
                    end = i;
                }
            }
            i += 1;
        }
    }

    // No balanced substring
    if (start == -1 || end == -1)
        System.out.println(-1);

    // Store answer string
    else 
    {
        String ans = "";
        for(int j = start; j <= end; j++)
            ans += s.charAt(j);
            
        System.out.println(ans);
    }
}

// Driver Code
public static void main(String[] args)
{
    
    // Given string
    String s = "azABaabba";

    smallestBalancedSubstring(s);
}
}

// This code is contributed by Dharanendra L V