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@@ -0,0 +1,40 @@
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+package main
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+
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+import "fmt"
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+
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+var chainLengths = make(map[int]int)
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+
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+func main() {
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+ var maxChainLength int
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+ var maxStarting int
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+ for i := 1; i < 1e6; i++ {
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+ l := ChainLength(i)
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+ if l > maxChainLength {
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+ maxChainLength = l
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+ maxStarting = i
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+ }
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+ }
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+ fmt.Println(maxStarting)
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+}
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+
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+func Collatz(n int) int {
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+ if n%2 == 0 {
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+ return n / 2
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+ } else {
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+ return 3*n + 1
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+ }
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+}
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+
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+func ChainLength(n int) int {
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+ l := 0
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+ for n != 1 {
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+ i, ok := chainLengths[n]
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+ if ok {
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+ return l + i
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+ }
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+ n = Collatz(n)
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+ l++
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+ }
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+ chainLengths[n] = l
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+ return l + 1
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+}
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