LeetCode/java/k-th-smallest-in-lexicographical-order.java

class Solution {
  private int distance(int upperBound, long left, long right) {
    var dist = 0;

    for (; left <= upperBound; left *= 10, right *= 10) {
      dist += Math.min(upperBound + 1, right) - left;
    }

    return dist;
  }

  public int findKthNumber(int n, int k) {
    var current = 1;
    --k;

    while (k > 0) {
      var d = distance(n, current, current + 1);

      if (d <= k) {
        // skipping d numbers at once
        ++current;
        k -= d;
      } else {
        current *= 10;
        --k;
      }
    }

    return current;
  }
}