Java实现数组元素的排列组合

组合

组合是非递归实现的（有Bug，没有输出所以的组合）

package D919;

import java.util.ArrayList;
import java.util.List;

public class Combine {
    public static List> sub(int[] a) {
        int n = a.length;
        List> result = new ArrayList>();
        for (int i = 0; i < n; i++) {
            for (int j = i; j < n; j++) {
                List list = new ArrayList<>();
                for (int k = i; k <=j; k++) {
                    list.add(a[k]);
                }
                result.add(list);
            }
        }
        return result;
    }

    public static void main(String[] args) {
        int[] a = {1, -2, 4, 8, -4, 7, -1, -5};
        System.out.println(sub(a));
    }
}

递归实现

package D920;

import java.util.ArrayList;
import java.util.List;

public class Combine {
    static List list = new ArrayList<>();

    public static void sub(String s) {
        char[] chars = s.toCharArray();
        StringBuffer sb = new StringBuffer("");
        for (int i = 1; i <= chars.length; i++) {
            sub(chars, 0, i, sb);
        }
    }

    private static void sub(char[] c, int begin, int len, StringBuffer sb) {

        if (len == 0) {
            list.add(String.valueOf(sb));
            return;
        }
        if (begin == c.length) {
            return;
        }
        sb.append(c[begin]);
        sub(c, begin + 1, len - 1, sb);
        sb.deleteCharAt(sb.length() - 1);
        sub(c, begin + 1, len, sb);
    }

    public static void main(String[] args) {
        String s = "abc";
        sub(s);
        System.out.println(list);
    }
}

排列

排列是递归实现的

package D919;

public class Permute {
    public void permutation(int[] a, int length, int index) {
        if (index == length) {
            for (int i = 0; i < length; i++) {
                System.out.print(a[i] + " ");
            }
            System.out.println();
        } else {
            for (int i = index; i < length; i++) {
                if (isSwap(a, length, i)) {
                    swap(a, index, i);
                    permutation(a, length, index + 1);
                    swap(a, index, i);
                }
            }
        }
    }

    private boolean isSwap(int[] a, int length, int index) {
        for (int i = index + 1; i < length; i++) {
            if (a[index] == a[i]) {
                return false;
            }
        }
        return true;
    }

    private void swap(int[] a, int index, int i) {
        int temp = a[index];
        a[index] = a[i];
        a[i] = temp;
    }

    public static void main(String[] args) {
        int[] b = {1, 2, 3};
        new permute().permutation(b, b.length, 0);
    }
}