Ian Dorian Macleod
Problem 14
Problem Statement

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.

Approach

C++ implementation

#include <stdc++.h>
using namespace std;

int longestCollatz(int n) {
int num = 0;
int maxlen = 0;
vector cache(n+1);
for(int i = 0; i < cache.size(); i++) {
cache[i] = -1;
}
cache = 1;
for(int i = 2; i <= n; i++) {
long sequence = i;
int k = 0;
while (sequence != 1 && sequence >= i) {
k++;
if ((sequence % 2) == 0) sequence /= 2;
else sequence = (sequence * 3) + 1;
}
cache[i] = k + cache[sequence];

if (cache[i] > maxlen) {
maxlen = cache[i];
num = i;
}
}
return num;
}

int main() {
int ans = longestCollatz(1000000);
cout << ans << endl;
return 0;
}


You can also download the source code for this problem here and compile it on your local machine.
Further Analysis