Longest Collatz sequence under one million

collatz.go

package main

func main() {
	onem := 1000000
	visited := map[int]int{}
	maxstep := 1
	var maxn int

	for i := 1; i <= onem; i++ {
		n := i
		if _, ok := visited[n]; !ok {
			steps := 0
			for {
				if n <= 1 {
					break
				}
				if _, ok := visited[n]; !ok {
					n = collatz(n)
					steps++
				} else {
					steps += visited[n]
					break
				}
			}
			if steps > maxstep {
				maxstep = steps
				maxn = i
			}

			visited[i] = steps
		}
	}

	println(maxn, visited[maxn]+1)
}

func collatz(n int) int {
	if n%2 == 0 {
		return n / 2
	}

	return n*3 + 1
}