n的阶乘-递归-动态规划

递归：

public class Factorial {
    public int factorial(int n){
        if(n<0)
            return -1;
        else if(n==0)
            return 1;
        return n*factorial(n-1);
    }
    public static void main(String[] args){
        Factorial fac=new Factorial();
        System.out.print(fac.factorial(5));
    }
}

动态规划
自顶向下：

public class Factorial {
    public static int[] result;
    public int solve(int n){
        if(result[n]>=0)
            return result[n];
        result[n]=n*solve(n-1);
        return result[n];
    }
    public int factorial(int n){
        if(n<0)
            return -1;
        else if(n==0)
            return 1;
        result=new int[n+1];
        result[0]=1;
        for(int i=1;i<result.length;i++){
            result[i]=-1;
        }
        return solve(n);
    }
    public static void main(String[] args){
        Factorial fac=new Factorial();
        System.out.print(fac.factorial(5));
    }
}

自底向上：

public class Factorial {
    public int factorial(int n){
        if(n<0)
            return -1;
        else if(n==0)
            return 1;
        int[] result=new int[n+1];
        result[0]=1;
        for(int i=1;i<result.length;i++){
            result[i]=i*result[i-1];
        }
        return result[result.length-1];
    }
    public static void main(String[] args){
        Factorial fac=new Factorial();
        System.out.print(fac.factorial(5));
    }
}