feat(440): Hard

Given two integers n and k, return the kth lexicographically smallest integer in the range [1, n].

Example 1:

Input: n = 13, k = 2
Output: 10
Explanation: The lexicographical order is [1, 10, 11, 12, 13, 2, 3, 4, 5, 6, 7, 8, 9], so the second smallest number is 10.
Example 2:

Input: n = 1, k = 1
Output: 1

Constraints:

1 <= k <= n <= 109

internal/Leetcode/440/440.go

@@ -0,0 +1,29 @@
+package _440
+
+func findKthNumber(n int, k int) int {
+	current := 1
+	k -= 1 // ponieważ zaczynamy od pierwszej liczby
+
+	for k > 0 {
+		steps := calculateSteps(current, current+1, n)
+		if steps <= k {
+			current += 1
+			k -= steps
+		} else {
+			current *= 10
+			k -= 1
+		}
+	}
+
+	return current
+}
+
+func calculateSteps(curr, next, n int) int {
+	steps := 0
+	for curr <= n {
+		steps += min(n+1, next) - curr
+		curr *= 10
+		next *= 10
+	}
+	return steps
+}

internal/Leetcode/440/440_test.go

@@ -0,0 +1,48 @@
+package _440
+
+import (
+	"testing"
+)
+
+func Test_findKthNumber(t *testing.T) {
+	type args struct {
+		n int
+		k int
+	}
+	tests := []struct {
+		name string
+		args args
+		want int
+	}{
+		{
+			name: "Example 1",
+			args: args{
+				n: 13,
+				k: 2,
+			},
+			want: 10,
+		},
+		{
+			name: "Example 2",
+			args: args{
+				n: 10,
+				k: 3,
+			},
+			want: 2,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := findKthNumber(tt.args.n, tt.args.k); got != tt.want {
+				t.Errorf("findKthNumber() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
+
+func BenchmarkFindKthNumber(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		findKthNumber(4089173, 3098723)
+	}
+}