440 - K-th Smallest in Lexicographical Order

 Description :-

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

Solutions:- 

C++

• class Solution {
  public:
      int n;
  
      int findKthNumber(int n, int k) {
          this->n = n;
          --k;
          long long curr = 1;
          while (k) {
              int cnt = count(curr);
              if (k >= cnt) {
                  k -= cnt;
                  ++curr;
              } else {
                  --k;
                  curr *= 10;
              }
          }
          return (int) curr;
      }
  
      int count(long long curr) {
          long long next = curr + 1;
          int cnt = 0;
          while (curr <= n) {
              cnt += min(n - curr + 1, next - curr);
              next *= 10;
              curr *= 10;
          }
          return cnt;
      }
  };

JAVA

• class Solution {
      private int n;
  
      public int findKthNumber(int n, int k) {
          this.n = n;
          long curr = 1;
          --k;
          while (k > 0) {
              int cnt = count(curr);
              if (k >= cnt) {
                  k -= cnt;
                  ++curr;
              } else {
                  --k;
                  curr *= 10;
              }
          }
          return (int) curr;
      }
  
      public int count(long curr) {
          long next = curr + 1;
          long cnt = 0;
          while (curr <= n) {
              cnt += Math.min(n - curr + 1, next - curr);
              next *= 10;
              curr *= 10;
          }
          return (int) cnt;
      }
  }