Project Euler Question 14 (Collatz Problem)

Question

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 wit

Answer 1

I solved the problem some time ago and luckily still have my code. Do not read the code if you don't want a spoiler:

#include 

int lookup[1000000] = { 0 };

unsigned int NextNumber(unsigned int value) {
  if ((value % 2) == 0) value >>= 1;
  else value = (value * 3) + 1;
  return value;
}

int main() {
  int i = 0;
  int chainlength = 0;
  int longest = 0;
  int longestchain = 0;
  unsigned int value = 0;
  for (i = 1; i < 1000000; ++i) {
    chainlength = 0;
    value = i;
    while (value != 1) {
      ++chainlength;
      value = NextNumber(value);
      if (value >= 1000000) continue;
      if (lookup[value] != 0) {
        chainlength += lookup[value];
        break;
      }
    }

    lookup[i] = chainlength;

    if (longestchain < chainlength) {
      longest = i;
      longestchain = chainlength;
    }
  }
  printf("\n%d: %d\n", longest, longestchain);
}

time ./a.out

[don't be lazy, run it yourself]: [same here]

real    0m0.106s
user    0m0.094s
sys     0m0.012s