算法 之 递归

public class T5_1
{
    public static void test(int[] x)
    {
        int a = x[0]*1000 + x[1]*100 + x[2]*10 + x[3];
        int b = x[4]*10000 + x[5]*1000 + x[6]*100 + x[7]*10 + x[8];      
        if(a*3==b) System.out.println(a + " " + b);
    }

    public static void f(int[] x, int k)
    {
        if(k>=x.length){
            test(x);
            return;
        }

        for(int i=k; i<x.length; i++){
            {int t=x[k]; x[k]=x[i]; x[i]=t;}
            f(x,k+1);

            int t=x[k]; x[k]=x[i]; x[i]=t;       // 填空
        }
    }

    public static void main(String[] args)
    {
        int[] x = {1,2,3,4,5,6,7,8,9};       
        f(x,0);
    }
}

class T3_A {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        int a[] = new int[8];
        boolean visit[] = new boolean[10];
        dfs(a,visit,0);
        System.out.println("end");
    }

    private static void dfs(int[] a, boolean[] visit, int i) {
        // TODO Auto-generated method stub
        if (i==8) {
            judge(a);
            return;
        }

        for (a[i] = 0; a[i] < visit.length; a[i]++) {
            if (i==4&&a[4]!=1) {
                continue;
            }
            if (visit[a[i]]==false) {
                visit[a[i]]=true;
                i = i + 1;

//             for(int j=0;j<a.length;j++){
//                 System.out.println("a["+j+"]="+a[j]+",v["+j+"]="+visit[j]);
//
//             }
                dfs(a, visit, i);

                i = i - 1;
                visit[a[i]]=false;
            }
        }
    }

    private static void judge(int[] a) {
        // TODO Auto-generated method stub
        int up = a[0]*1000+a[1]*100+a[2]*10+a[3];
        int down = a[4]*1000+a[5]*100+a[6]*10+a[1];
        int answer = a[4]*10000+a[5]*1000+a[2]*100+a[1]*10+a[7];
        if (up+down==answer) {
            System.out.println(down);//1085
        }
    }

}