A: First, let’s look at a simple interview question
Fibonacci Sequence
Rule: 1 1 Starting from the third item, each item is the sum of the previous two items
There are two ways to implement it
The first way:
TestSelf((n<0 IllegalArgumentException("n不能为负数" (n<=2 1 TestSelf(n-2)+TestSelf(n-1 30
The second way: using array
public int TestSelfTwo(int n){ if(n<0){ throw new IllegalArgumentException("n不能为负数"); }else if(n<=1){ //递归前两个数 不管n是多少 为一 return 1; } int[] nums = new int[n+1]; //30位从零开始 nums[0]=1; nums[1]=1; for (int i =2;i<n;i++){ nums[i] = nums[i-2]+nums[i-1]; } return nums[n-1]; } @Test public void Test(){ System.out.println(TestSelfTwo(30)); }
Formula: f(n) = f(n-2)+f(n-1) f represents the number of bits represented by method n
B: Use recursion to implement n-level linkage in MyBatis
sql statement: select * from type where pid = 0; Specify the pid value as 0 for the first time, and then use the cid with pid as 0 as the next query pid
public List<Category> getCategory(Integer pid); //接口层方法
<mapper namespace="dao.CateGoryDao"><resultMap id="getSelf" type="entity.Category"><id column="cid" property="cid"></id><result column="cname" property="cName"></result><collection property="categorySet" select="getCategory" column="cid"></collection> //这里可以不用指定oftype 使用反向查询select从另一个maper文件中取出数据时必须用ofType<!--查到的cid作为下次的pid--></resultMap><select id="getCategory" resultMap="getSelf" >select * from category where pid=#{pid}</select></mapper>
are all types of specified objects. The difference is that when using reverse query select to retrieve data from another maper file, you must use ofType
. The type of object can be specified for collection and association.
is not required to be written, only ofType is required for reverse selection;
Entity class:
package entity;import java.util.HashSet;import java.util.Set;/** * Created by zhangyu on 2017/7/12. */public class Category {private Integer cid;private String cName;private Integer pid;private Set<Category> categorySet = new HashSet<Category>(); @Overridepublic String toString() {return "Category{" + "cid=" + cid + ", cName='" + cName + '\'' + ", pid=" + pid + ", categorySet=" + categorySet + '}'; }public Integer getCid() {return cid; }public void setCid(Integer cid) {this.cid = cid; }public String getcName() {return cName; }public void setcName(String cName) {this.cName = cName; }public Integer getPid() {return pid; }public void setPid(Integer pid) {this.pid = pid; }public Set<Category> getCategorySet() {return categorySet; }public void setCategorySet(Set<Category> categorySet) {this.categorySet = categorySet; } }
Test class:
//测试自连接 @Testpublic void TestSelf(){ CateGoryDao dao = MyBatis.getSessionTwo().getMapper(CateGoryDao.class); List<Category> list = dao.getCategory(0);for (Category item:list ) { System.out.println(item); } }
Print result:
Category{cid=1, cName='图书', pid=0, categorySet=[Category{cid=5, cName='期刊报纸', pid=1, categorySet=[]}, Category{cid=3, cName='青年图书', pid=1, categorySet=[Category{cid=6, cName='读者', pid=3, categorySet=[Category{cid=7, cName='12月份', pid=6, categorySet=[]}]}]}, Category{cid=4, cName='少儿图书', pid=1, categorySet=[]}]} Category{cid=2, cName='服装', pid=0, categorySet=[]}
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