[백준][Java] 16205번 변수명

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Scanner sc = new Scanner(System.in);
        
        int N = sc.nextInt(); 
        String s = sc.next();
        
        switch (N) {
        case 1:
            String s2 = "";
            for(int i=0; i<s.length(); i++) {
                if('A'<=s.charAt(i) && s.charAt(i)<='Z') {
                    s2+=("_"+String.valueOf(s.charAt(i)).toLowerCase());
                }
                else {
                    s2+=String.valueOf(s.charAt(i));
                }
            }
            System.out.println(s);
            System.out.println(s2);
            System.out.println(String.valueOf(s.charAt(0)).toUpperCase()+s.substring(1));
            
            break;
        case 2:
            String s1 = "";
            boolean toUpper = false;
            for(int i=0; i<s.length(); i++) {
                if(s.charAt(i)=='_') {
                    toUpper = true;
                }
                else {
                    if(toUpper) {
                        s1+=String.valueOf(s.charAt(i)).toUpperCase();
                        toUpper=false;
                    }
                    else
                        s1+=String.valueOf(s.charAt(i));
                    
                }
            }
            System.out.println(s1);
            System.out.println(s);
            System.out.println(String.valueOf(s1.charAt(0)).toUpperCase()+s1.substring(1)); 
            
            break;
        case 3:
            
            String finS2="";
            for(int i=0; i<s.length(); i++) {
                if('A'<=s.charAt(i) && s.charAt(i)<='Z') {
                    if(i==0)  
                        finS2+=(String.valueOf(s.charAt(i)).toLowerCase());
                    else
                        finS2+=("_"+String.valueOf(s.charAt(i)).toLowerCase());
                }
                else {
                    finS2+=String.valueOf(s.charAt(i));
                }
            }
            System.out.println(String.valueOf(s.charAt(0)).toLowerCase()+s.substring(1));
            System.out.println(finS2);
            System.out.println(s);
            
            break;
        
        }
        
    }