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package code_07_dp;
import org.junit.Test;
/**
* Created by 18351 on 2018/11/8.
*/
public class Fib {
public int fib(int n){
if(n==0){
return 0;
}
if(n==1){
return 1;
}
return fib(n-1)+fib(n-2);
}
//改进上面的方法：采用记忆化搜索
private int[] memo;
public int fib2(int n){
if(n==0){
return 0;
}
if(n==1){
return 1;
}
if(memo[n]==-1){
//memo[n]==-1说明是首次计算，下一次遇到该值就直接返回了
memo[n]=fib2(n-1)+fib2(n-2);
}
return memo[n];
}
@Test
public void test(){
//int n=10; //time:2.0921E-5s
//int n=20; //time:0.009386947s
//int n=30; //time:0.049079365s
//int n=40;//time:1.157836107s
int n=42; //time:2.560889825s
//可以看到时间是成指数级增长的
long startTime=System.nanoTime();
int res=fib(n);
long endTime=System.nanoTime();
System.out.println("fib("+n+")="+res);
System.out.println("time:"+(endTime-startTime)/1000000000.0+"s");
}
@Test
public void test2(){
//int n=10; //time:7.106E-6s
//int n=20; //time:8.289E-6s
//int n=30; //time:2.0132E-5s
//int n=40;//time:1.2632E-5s
int n=42; //time:1.1843E-5s
//为memo分配空间，并且初始值设为-1,因为斐波那契数列中是没有-1的
memo=new int[n+1];
for(int i=0;i<n+1;i++){
memo[i]=-1;
}
long startTime=System.nanoTime();
int res=fib2(n);
long endTime=System.nanoTime();
System.out.println("fib2("+n+")="+res);
System.out.println("time:"+(endTime-startTime)/1000000000.0+"s");
}
}