Java 조합과 순열

public class test{
    public static void main(String[] args) {
        int[] arr = {1, 3, 5, 7, 9};
        boolean[] visit = new boolean[5];
        combination(visit, 5, 3, 0, arr);
    }
 
    private static void combination(boolean[] visit, int n, int r, int start, int[] arr) {
        if (r == 0) {
            for (int i = 0; i < visit.length; i++) {
                if(visit[i]) System.out.print(arr[i]+" ");
            }
            System.out.println();
            return;
        }
        for (int i = start; i < n; i++) {
            visit[i] = true;
            combination(visit, n, r - 1, i + 1, arr);
            visit[i] = false;
        }
    }
}
1 3 5  
1 3 7  
1 3 9  
1 5 7  
1 5 9  
1 7 9  
3 5 7  
3 5 9  
3 7 9  
5 7 9

 

조합

nCr   n개 중 r개를 뽑음. 

 

 

public class test{
    public static void main(String[] args) {
        int[] arr = {1, 3, 5, 7, 9};
        int[] combArr = new int[3];
        boolean[] visit = new boolean[5];
//        combination(visit, 5, 3, 0, arr);
        permutation(visit, 5, 3,  arr, combArr, 0);
    }
 
    private static void permutation(boolean[] visit, int n, int r, int[] arr, int[] combArr, int index) {
        if (r == index) {
            for (int i = 0; i < 3; i++) {
                System.out.print(combArr[i]+" ");
            }
            System.out.println();
            return;
        }
 
        for (int i = 0; i < n; i++) {
            if(visit[i]) continue;
            combArr[index] = arr[i];
            visit[i] = true;
            permutation(visit, n, r, arr, combArr, index + 1);
            visit[i] = false;
        }
    }
}
1 3 5  1 3 7  1 3 9  1 5 3  1 5 7  1 5 9  1 7 3  1 7 5  1 7 9  1 9 3  1 9 5  1 9 7 
3 1 5  3 1 7  3 1 9  3 5 1  3 5 7  3 5 9  3 7 1  3 7 5  3 7 9  3 9 1  3 9 5  3 9 7 
5 1 3  5 1 7  5 1 9  5 3 1  5 3 7  5 3 9  5 7 1  5 7 3  5 7 9  5 9 1  5 9 3  5 9 7 
7 1 3  7 1 5  7 1 9  7 3 1  7 3 5  7 3 9  7 5 1  7 5 3  7 5 9  7 9 1  7 9 3  7 9 5 
9 1 3  9 1 5  9 1 7  9 3 1  9 3 5  9 3 7  9 5 1  9 5 3  9 5 7  9 7 1  9 7 3  9 7 5
60

순서를 신경써가면서 뽑음 nPr 

 

swap을 이용한 방법도 있음

 

public class test{
    public static void main(String[] args) {
        int[] arr = {1, 2, 3, 4, 5};
        permutation2(arr, 0, 5, 3);
    }
 
    private static void permutation2(int[] arr, int depth, int n, int r) {
        if (depth == r) {
            for (int i = 0; i < r; i++) {
                System.out.print(arr[i]+" ");
            }
            System.out.println();
            return;
        }
        for (int i = depth; i < n; i++) {
            swap(arr, depth, i);
            permutation2(arr, depth + 1, n, r);
            swap(arr, depth, i);
        }
    }
 
    private static void swap(int[] arr, int depth, int i) {
        int temp = arr[depth];
        arr[depth] = arr[i];
        arr[i] = temp;
    }
}