_440.java

package com.fishercoder.solutions;

/**
 * 440. K-th Smallest in Lexicographical Order
 *
 * Given integers n and k, find the lexicographically k-th smallest integer in the range from 1 to n.

 Note: 1 ≤ k ≤ n ≤ 109.

 Example:

 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.
 */
public class _440 {

    /**credit: https://discuss.leetcode.com/topic/64624/concise-easy-to-understand-java-5ms-solution-with-explaination/2*/
    public int findKthNumber(int n, int k) {
        int curr = 1;
        k = k - 1;
        while (k > 0) {
            int steps = calSteps(n, curr, curr + 1);
            if (steps <= k) {
                curr += 1;
                k -= steps;
            } else {
                curr *= 10;
                k -= 1;
            }
        }
        return curr;
    }

    //use long in case of overflow
    public int calSteps(int n, long n1, long n2) {
        int steps = 0;
        while (n1 <= n) {
            steps += Math.min(n + 1, n2) - n1;
            n1 *= 10;
            n2 *= 10;
        }
        return steps;
    }

}