go: add «264. Ugly Number II»

Signed-off-by: Matej Focko <me@mfocko.xyz>

go/ugly-number-ii.go

@@ -0,0 +1,50 @@
+package main
+
+import (
+	"cmp"
+	"slices"
+)
+
+type Factor struct {
+	factor int
+	index  int
+	next   int
+}
+
+func MakeFactor(factor int) Factor {
+	return Factor{
+		factor: factor,
+		index:  0,
+		next:   factor,
+	}
+}
+
+func (f *Factor) Update(ugly []int, last int) {
+	if last != f.next {
+		return
+	}
+
+	f.index++
+	f.next = f.factor * ugly[f.index]
+}
+
+func nthUglyNumber(n int) int {
+	ugly := make([]int, n)
+	ugly[0] = 1
+
+	factors := []Factor{
+		MakeFactor(2), MakeFactor(3), MakeFactor(5),
+	}
+
+	for i := 1; i < n; i++ {
+		ugly[i] = slices.MinFunc(factors, func(a, b Factor) int {
+			return cmp.Compare(a.next, b.next)
+		}).next
+
+		for j, _ := range factors {
+			factors[j].Update(ugly, ugly[i])
+		}
+	}
+
+	return ugly[n-1]
+}