我需要一些帮助,我正在为Universiy的Programming II课程编写一个程序。问题是要求使用递归计算Fibonacci序列。必须将计算出的斐波纳契数存储在一个数组中,以阻止不必要的重复计算并减少计算时间。
我设法让程序在没有数组和记忆的情况下工作,现在我正在尝试实现它并且我被卡住了。我不确定如何构建它。我用谷歌搜索并略读了一些书,但没有找到太多帮助我解决如何实施解决方案。
import javax.swing.JOptionPane;
public class question2
{
static int count = 0;
static int [] dictionary;
public static void main(String[] args)
{
int answer;
int num = Integer.parseInt(javax.swing.JOptionPane.showInputDialog("Enter n:"));
javax.swing.JOptionPane.showMessageDialog(null,
"About to calculate fibonacci(" + num + ")");
//giving the array "n" elements
dictionary= new int [num];
if (dictionary.length>=0)
dictionary[0]= 0;
if (dictionary.length>=1)
dictionary[0]= 0;
dictionary[1]= 1;
//method call
answer = fibonacci(num);
//output
JOptionPane.showMessageDialog(null,"Fibonacci("+num+") is "+answer+" (took "+count+" calls)");
}
static int fibonacci(int n)
{
count++;
// Only defined for n >= 0
if (n < 0) {
System.out.println("ERROR: fibonacci sequence not defined for negative numbers.");
System.exit(1);
}
// Base cases: f(0) is 0, f(1) is 1
// Other cases: f(n) = f(n-1) + f(n-2)/
if (n == 0)
{
return dictionary[0];
}
else if (n == 1)
{
return dictionary[1];
}
else
return dictionary[n] = fibonacci(n-1) + fibonacci(n-2);
}
}
以上是不正确的,我的fib方法的结束是主要问题。我不知道如何将数字递归地添加到数组的正确部分。
您需要区分字典中已计算的数字和未计算的数字,而您目前不需要:您总是重新计算数字。
if (n == 0)
{
// special case because fib(0) is 0
return dictionary[0];
}
else
{
int f = dictionary[n];
if (f == 0) {
// number wasn't calculated yet.
f = fibonacci(n-1) + fibonacci(n-2);
dictionary[n] = f;
}
return f;
}
#include <stdio.h>
long int A[100]={1,1};
long int fib(int n){
if (A[n])
{
return A[n];
}
else
{
return A[n]=fib(n-1)+fib(n-2);
}
}
int main(){
printf("%ld",fib(30));
}
这是我的实施。
private static int F(int N, int[] A) {
if ((N == 0) || (N == 1)) return N;
if (A[N] != 0) return A[N];
if ((A[N - 1] != 0) && (A[N - 2] != 0)) {
A[N] = A[N - 1] + A[N - 2];
return A[N];
}
if (A[N-2] != 0) {
A[N] = A[N - 2] + F(N - 1, A);
return A[N];
}
if (A[N-1] != 0) {
A[N] = A[N - 1] + F(N - 2, A);
return A[N];
}
A[N] = F(N-1, A) + F(N-2, A);
return A[N];
}
public static int fib(int n, Map<Integer,Integer> map){
if(n ==0){
return 0;
}
if(n ==1){
return 1;
}
if(map.containsKey(n)){
return map.get(n);
}
Integer fibForN = fib(n-1,map) + fib(n-2,map);
map.put(n, fibForN);
return fibForN;
}
与上述大多数解决方案类似,但使用地图代替。
程序使用Memoization打印第一个n
斐波那契数字。
int[] dictionary;
// Get Fibonacci with Memoization
public int getFibWithMem(int n) {
if (dictionary == null) {
dictionary = new int[n];
}
if (dictionary[n - 1] == 0) {
if (n <= 2) {
dictionary[n - 1] = n - 1;
} else {
dictionary[n - 1] = getFibWithMem(n - 1) + getFibWithMem(n - 2);
}
}
return dictionary[n - 1];
}
public void printFibonacci()
{
for (int curr : dictionary) {
System.out.print("F[" + i++ + "]:" + curr + ", ");
}
}
我相信你忘了在字典中查找实际内容。
更改
else
return dictionary[n] = fibonacci(n-1) + fibonacci(n-2);
至
else {
if (dictionary[n] > 0)
return dictionary[n];
return dictionary[n] = fibonacci(n - 1) + fibonacci(n - 2);
}
它工作得很好(自己测试:)
这是我对递归斐波那契记忆的实现。使用BigInteger和ArrayList可以计算出第100个甚至更长的项。我尝试了第1000个术语,结果在几毫秒内返回,这里是代码:
private static List<BigInteger> dict = new ArrayList<BigInteger>();
public static void printFebonachiRecursion (int num){
if (num==1){
printFebonachiRecursion(num-1);
System.out.printf("Term %d: %d%n",num,1);
dict.add(BigInteger.ONE);
}
else if (num==0){
System.out.printf("Term %d: %d%n",num,0);
dict.add(BigInteger.ZERO);
}
else {
printFebonachiRecursion(num-1);
dict.add(dict.get(num-2).add(dict.get(num-1)));
System.out.printf("Term %d: %d%n",num,dict.get(num));
}
}
输出示例
printFebonachiRecursion(100);
Term 0: 0
Term 1: 1
Term 2: 1
Term 3: 2
...
Term 98: 135301852344706746049
Term 99: 218922995834555169026
Term 100: 354224848179261915075
int F(int Num){
int i =0;
int* A = NULL;
if(Num > 0)
{
A = (int*) malloc(Num * sizeof(int));
}
else
return Num;
for(;i<Num;i++)
A[i] = -1;
return F_M(Num, &A);
}
int F_M(int Num, int** Ap){
int Num1 = 0;
int Num2 = 0;
if((*Ap)[Num - 1] < 0)
{
Num1 = F_M(Num - 1, Ap);
(*Ap)[Num -1] = Num1;
printf("Num1:%d\n",Num1);
}
else
Num1 = (*Ap)[Num - 1];
if((*Ap)[Num - 2] < 0)
{
Num2 = F_M(Num - 2, Ap);
(*Ap)[Num -2] = Num2;
printf("Num2:%d\n",Num2);
}
else
Num2 = (*Ap)[Num - 2];
if(0 == Num || 1 == Num)
{
(*Ap)[Num] = Num;
return Num;
}
else{
// return ((*Ap)[Num - 2] > 0?(*Ap)[Num - 2] = F_M(Num -2, Ap): (*Ap)[Num - 2] ) + ((*Ap)[Num - 1] > 0?(*Ap)[Num - 1] = F_M(Num -1, Ap): (*Ap)[Num - 1] );
return (Num1 + Num2);
}
}
int main(int argc, char** argv){
int Num = 0;
if(argc>1){
sscanf(argv[1], "%d", &Num);
}
printf("F(%d) = %d", Num, F(Num));
return 0;
}
这是使用静态数组值来处理递归fibonacci()方法的memoization的另一种方法 -
public static long fibArray[]=new long[50];\\Keep it as large as you need
public static long fibonacci(long n){
long fibValue=0;
if(n==0 ){
return 0;
}else if(n==1){
return 1;
}else if(fibArray[(int)n]!=0){
return fibArray[(int)n];
}
else{
fibValue=fibonacci(n-1)+fibonacci(n-2);
fibArray[(int) n]=fibValue;
return fibValue;
}
}
请注意,此方法使用全局(类级别)静态数组fibArray []。要查看整个代码并附带解释,您还可以看到以下内容 - http://www.javabrahman.com/gen-java-programs/recursive-fibonacci-in-java-with-memoization/
这是一个充分利用memoization概念的完全成熟的类:
import java.util.HashMap;
import java.util.Map;
public class Fibonacci {
public static Fibonacci getInstance() {
return new Fibonacci();
}
public int fib(int n) {
HashMap<Integer, Integer> memoizedMap = new HashMap<>();
memoizedMap.put(0, 0);
memoizedMap.put(1, 1);
return fib(n, memoizedMap);
}
private int fib(int n, Map<Integer, Integer> map) {
if (map.containsKey(n))
return map.get(n);
int fibFromN = fib(n - 1, map) + fib(n - 2, map);
// MEMOIZE the computed value
map.put(n, fibFromN);
return fibFromN;
}
}
请注意
memoizedMap.put(0, 0);
memoizedMap.put(1, 1);
用于消除以下检查的必要性
if (n == 0) return 0;
if (n == 1) return 1;
在每个递归函数调用。
import java.util.HashMap;
import java.util.Map;
public class FibonacciSequence {
public static int fibonacci(int n, Map<Integer, Integer> memo) {
if (n < 2) {
return n;
}
if (!memo.containsKey(n)) {
memo.put(n, fibonacci(n - 1, memo) + fibonacci(n - 2, memo));
}
return memo.get(n);
}
public static int fibonacci(int n, int[] memo) {
if (n < 2) {
return n;
}
if (memo[n - 1] != 0) {
return memo[n - 1];
}
return memo[n - 1] = fibonacci(n - 1, memo) + fibonacci(n - 2, memo);
}
public static void main(String[] s) {
int n = 10;
System.out.println("f(n) = " + fibonacci(n, new HashMap<Integer, Integer>()));
System.out.println("f(n) = " + fibonacci(n, new int[n]));
}
}