Project Euler – Problem 14

The following iterative sequence is defined for the set of positive integers:

n n/2 (n is even)

n 3n + 1 (n is odd)

Using the rule above and starting with 13, we generate the following sequence:

13 40 20 10 5 16 8 4 2 1

It can be seen that this sequence (starting at 13 and finishing at 1) contains 10 terms. Although it has not been proved yet (Collatz Problem), it is thought that all starting numbers finish at 1.

Which starting number, under one million, produces the longest chain?

NOTE: Once the chain starts the terms are allowed to go above one million.

用一个大的数组存储计算结果，遍历计算其后的长度~~~

namespace Problem14
{
class Program
{
static int[] Res = new int[1000001];

static void Main(string[] args)
{
int ans;
ans = Res[1] = 1;
for (int i = 2; i <= 1000000; i++)
{
if (Res[i] == 0)
Calc(i);
if (Res[i] > Res[ans])
ans = i;
}
Console.WriteLine(ans);
Console.Read();
}

static int Calc(long n)
{
if (n < 1000000)
{
if (Res
== 0)
{
if (n % 2 == 0)
Res
= Calc((long)n / 2) + 1;
else
Res
= Calc(3 * n + 1) + 1;
}
return Res
;
}

if (n % 2 == 0)
return Calc((long)n / 2) + 1;
else
return Calc(3 * n + 1) + 1;

}
}
}