24: lexicographic permutations

08_largest_product_in_a_series/main.go

@@ -3,6 +3,7 @@ package main
 import (
 	"fmt"
 	"strconv"
+	"time"
 )
 
 var src = "73167176531330624919225119674426574742355349194934" +
@@ -36,8 +37,10 @@ func main() {
 		nextSrc = append(nextSrc, i)
 		src = src[1:]
 	}
+	t := time.Now()
 	var next []int
 	var out int
+	var outS []int
 	for {
 		next, nextSrc = FetchNextAdjacent(13, nextSrc)
 		if len(next) == 0 {
@@ -50,10 +53,12 @@ func main() {
 		}
 		if product > out {
 			out = product
+			outS = next
 		}
 	}
 
-	fmt.Println("the largest product is", out)
+	fmt.Println("time:", time.Since(t))
+	fmt.Println("the largest product is", out, outS)
 }
 
 func FetchNextAdjacent(n int, src []int) ([]int, []int) {

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
+}

24_lexicographic_permutations/main_test.go

@@ -0,0 +1,109 @@
+package main
+
+import (
+	"errors"
+	"reflect"
+	"testing"
+)
+
+func Test_findPermutation(t *testing.T) {
+	type args struct {
+		n int
+		m int
+	}
+	tt := []struct {
+		name    string
+		args    args
+		want    int
+		wantErr error
+	}{
+		{
+			"1-2",
+			args{1, 2},
+			0,
+			errors.New("m=2 cannot be larger than n=1"),
+		},
+		{
+			"1-1",
+			args{1, 1},
+			1,
+			nil,
+		},
+		{
+			"3-3",
+			args{3, 3},
+			6,
+			nil,
+		},
+		{
+			"4-4",
+			args{4, 4},
+			24,
+			nil,
+		},
+		{
+			"3-2",
+			args{3, 2},
+			6,
+			nil,
+		},
+		{
+			"4-2",
+			args{4, 2},
+			12,
+			nil,
+		},
+	}
+	for _, tc := range tt {
+		t.Run(tc.name, func(t *testing.T) {
+			got, err := findPermutation(tc.args.n, tc.args.m)
+			if !reflect.DeepEqual(err, tc.wantErr) {
+				t.Errorf("findPermutation() = %v, want err %v", err, tc.wantErr)
+			}
+			if got != tc.want {
+				t.Errorf("findPermutation() = %v, want %v", got, tc.want)
+			}
+		})
+	}
+}
+
+func Test_findNPermutation(t *testing.T) {
+	type args struct {
+		n    int
+		src  []int
+		facs []int
+	}
+	tests := []struct {
+		name string
+		args args
+		want string
+	}{
+		{
+			"012-1",
+			args{1, []int{0, 1, 2}, []int{1, 1, 2}},
+			"012",
+		},
+		{
+			"012-2",
+			args{2, []int{0, 1, 2}, []int{1, 1, 2}},
+			"021",
+		},
+		{
+			"012-6",
+			args{6, []int{0, 1, 2}, []int{1, 1, 2}},
+			"210",
+		},
+		{
+			"0123456-1",
+			args{1, []int{0, 1, 2, 3, 4, 5, 6}, []int{1, 1, 2, 6, 24, 120, 720}},
+			"0123456",
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := findNPermutation(tt.args.n, tt.args.src, tt.args.facs); got != tt.want {
+				t.Errorf("findNPermutation() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}