public class Permutation {    static int[] a;    public Permutation(int[] a){        this.a = a;    }    public static void main(String[] args) {        int a[] = new int[]{1,2,3,4};        Permutation p = new Permutation(a);        p.permutation(a.length,0);    }    private void permutation(int length, int start) {        if (start == length -1){            print(length);        } else {            for (int i = start; i< length;i++){        //for循环只有第一次进来是i = start                swap(i,start);                permutation(length,start+1);                swap(i,start);//注意这个地方是索引值,一定不要填写a[i]  a[start]            }        }    }    private void swap(int i, int start) {        int tmp = a[start];        a[start] = a[i];        a[i] = tmp;    }    private void print(int length) {        for (int i = 0; i < length;i++){            System.out.printf(a[i]+" ");        }        System.out.println();    }}