경우의 수( 순열 / 중복순열 / 조합 / 중복조합 / 부분집합 )

package basic;
 
import java.util.ArrayList;
import java.util.Arrays;
 
public class NumberOfCases {
    static char[] arr = { 'a', 'b', 'c', 'd' };
    static int r = 2;
 
    public static void main(String[] args) {
 
        set = new char[r];
        // System.out.println("==조합==");
        // nCr(0,0);
        //comb(r,arr.length);
        // System.out.println("==중복조합==");
        // nHr(0,0);
        //System.out.println("==중복순열==");
        //nPir(0);
        setList = new ArrayList<>();
        //subset(0,0);
        
        visit = new boolean[arr.length];
        nPr(0);
    }
 
    static char[] set;
 
    public static void nCr(int len, int k) { // 조합
        if (len == r) {
            System.out.println(Arrays.toString(set));
            return;
        }
 
        for (int i = k; i < arr.length; i++) {
            set[len] = arr[i];
            nCr(len + 1, i + 1);
        }
    }
 
    public static void comb(int depth, int idx) {
        if (depth==0) {
            System.out.println(Arrays.toString(set));
        } else if (idx < depth) {
            return;
        } else {
            //System.out.println(depth + " " + idx );
            set[depth-1] = arr[idx-1];
 
            comb(depth - 1, idx - 1);
            comb(depth, idx-1);
        }
 
    }
 
    public static void nHr(int len, int k) { // 중복조합
        if (len == r) {
            System.out.println(Arrays.toString(set));
            return;
        }
 
        for (int i = k; i < arr.length; i++) {
            set[len] = arr[i];
            nHr(len + 1, i);
        }
    }
    static boolean[] visit;
    public static void nPr(int len) {// 순열
        if(len==r) {
            System.out.println(Arrays.toString(set));
            return;
        }
        
        for(int i=0;i<arr.length;i++) {
            if(!visit[i]) {
                set[len]=arr[i];
                visit[i]=true;
                nPr(len+1);
                visit[i]=false;
            }
        }
    }
 
    public static void nPir(int len) {// 중복순열
        if(len==r) {
            System.out.println(Arrays.toString(set));
            return;
        }
        
        for(int i=0;i<arr.length;i++) {
            set[len]=arr[i];
            nPir(len+1);
        }
    }
 
    static ArrayList<Character> setList;
    public static void subset(int len, int k) {// 부분집합
        System.out.println(setList);
        if(len==arr.length) {
            return;
        }
        for(int i=k;i<arr.length;i++) {
            setList.add(arr[i]);
            subset(len+1,i+1);
            setList.remove(setList.size()-1);
        }
    }
}