Find Case Combinations of a String, Java

Find Case Combinations of a String

//'main' method must be in a class 'Rextester'.
//Compiler version 1.8.0_111

import java.util.*;
import java.lang.*;

class Rextester
{  
    public static void main(String args[])
    {
       String input = "aBc";
       List<String> res = caseCombination(input);
       System.out.println(res);
       input = "AirBnb";
       res = caseCombination(input);
       System.out.println(res + ", size: " + res.size());
       assert 64 == res.size();
       assert "airbnb" == res.get(0);
       assert "Airbnb" == res.get(1);
       assert "aIRBNB" == res.get(62);
       assert "AIRBNB" == res.get(63);
        
    }
    
    // Bit Manipulation
    private static List<String> caseCombination(String input) {
        List<String> res = new ArrayList<String>();
        if(input == null || input.length() == 0) return res;
        int len = input.length();
        char[] arr = input.toCharArray();
        for(int i = 0; i < (1 << len); i++) {
            StringBuilder sb = new StringBuilder();
            for(int j = 0; j < len; j++) {
                sb.append( (i >> j & 1)  != 0 ? Character.toUpperCase(arr[j]) : Character.toLowerCase(arr[j]));
            }
            res.add(sb.toString());
        }
        return res;
        
        
    }
    
}


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