24: lexicographic permutations

24_lexicographic_permutations/main.go

@@ -0,0 +1,42 @@
+package main
+
+import (
+	"fmt"
+	"strconv"
+)
+
+var nums = []int{0, 1, 2, 3, 4, 5, 6, 7, 8, 9}
+
+func main() {
+	var facs []int
+	for _, i := range nums {
+		n, _ := findPermutation(i, i)
+		facs = append(facs, n)
+	}
+	fmt.Println("facs:", facs)
+
+	fmt.Println("the millionth is:", findNPermutation(1e6, nums, facs))
+}
+
+func findNPermutation(n int, src []int, facs []int) string {
+	n--
+	var ret string
+	for i := len(src) - 1; i >= 0; i-- {
+		t := n / facs[i]
+		n %= facs[i]
+		ret += strconv.Itoa(src[t])
+		src = append(src[:t], src[t+1:]...)
+	}
+	return ret
+}
+
+func findPermutation(n, m int) (int, error) {
+	if m > n {
+		return 0, fmt.Errorf("m=%d cannot be larger than n=%d", m, n)
+	}
+	p := 1
+	for i := n - m + 1; i <= n; i++ {
+		p *= i
+	}
+	return p, nil
+}