20250609 finished.

src/problem/mod.rs

@@ -700,3 +700,5 @@ mod p2434_using_a_robot_to_print_the_lexicographically_smallest_string;
 mod p3170_lexicographically_minimum_string_after_removing_stars;
 
 mod p386_lexicographical_numbers;
+
+mod p440_k_th_smallest_in_lexicographical_order;

src/problem/p440_k_th_smallest_in_lexicographical_order.rs

@@ -0,0 +1,56 @@
+/**
+ * [440] K-th Smallest in Lexicographical Order
+ */
+pub struct Solution {}
+
+// submission codes start here
+
+impl Solution {
+    pub fn find_kth_number(n: i32, mut k: i32) -> i32 {
+        let mut currrent = 1;
+        let n = n as i64;
+        k -= 1;
+
+        while k > 0 {
+            let steps = Self::calculate_steps(currrent, n);
+            // If steps <= k, then the number is in the tree of next current (at least).
+            if steps <= k {
+                k -= steps;
+                currrent += 1;
+            } else {
+                // The number is in the current tree.
+                currrent = currrent * 10;
+                k -= 1;
+            }
+        }
+
+        currrent as i32
+    }
+
+    /// Calculate the count of number with prefix current.
+    fn calculate_steps(current: i64, n: i64) -> i32 {
+        let mut result = 0;
+        let (mut first, mut last) = (current, current);
+
+        while first <= n {
+            result += n.min(last) - first + 1;
+            first = first * 10;
+            last = last * 10 + 9;
+        }
+
+        result as i32
+    }
+}
+
+// submission codes end
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_440() {
+        assert_eq!(10, Solution::find_kth_number(13, 2));
+        assert_eq!(1, Solution::find_kth_number(1, 1));
+    }
+}