题目

题目链接：
https://www.nowcoder.com/practice/670c2bda374241d7ae06ade60de33e8b
https://leetcode.cn/problems/k-th-smallest-in-lexicographical-order/description/

本答案核心

10叉树,  数学规律

Java代码

import java.util.*;


public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     *
     * @param n int整型
     * @param k int整型
     * @return int整型
     */
    public int findKth (int n, int k) {
        //10叉树
        //https://blog.csdn.net/a1439775520/article/details/104913782
        int cur = 1;
        k--;

        while (k > 0) {

            long step = 0, first = cur, last = cur + 1;

            while (first <= n) {
                step += Math.min(last, (long)(n + 1)) - first;
                first *= 10;
                last *= 10;
            }

            if (step > k) { //在树里
                k--;
                cur *= 10;
            }

            if (step <= k) {

                //不在树里
                k -= step;
                cur++;
            }
        }

        return cur;
    }
}

Go代码

package main

/**
 * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
 *
 *
 * @param n int整型
 * @param k int整型
 * @return int整型
 */
func findKth(n int, k int) int {
	//10叉树
	//https://blog.csdn.net/a1439775520/article/details/104913782
	cur := 1
	k--

	for k > 0 {
		step := 0
		first := cur
		last := cur + 1

		for first <= n {
			num := n + 1
			if num > last {
				num = last
			}

			step += num - first
			first *= 10
			last *= 10
		}

		if step > k { //在树里
			k--
			cur *= 10
		}

		if step <= k { //不在树里
			k -= step
			cur += 1
		}
	}

	return cur
}

PHP代码

<?php


/**
 * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
 *
 * 
 * @param n int整型 
 * @param k int整型 
 * @return int整型
 */
function findKth( $n ,  $k )
{
    //10叉树
    //https://blog.csdn.net/a1439775520/article/details/104913782

    $cur =1;
    $k--;
    while ($k >0) {
        $step =0;
        $first =$cur;
        $last =$cur+1;

        while ($first<=$n){
            $num = $n+1;
            if($num >$last){
                $num=$last;
            }

            $step+= $num-$first;
            $first*=10;
            $last*=10;
        }

        if($step > $k){//在树里
            $k--;
            $cur*=10;
        }

        if($step<=$k){ //不在树里
            $k-=$step;
            $cur+=1;
        }
    }
    return $cur;
}

C++代码

class Solution {
  public:
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     *
     * @param n int整型
     * @param k int整型
     * @return int整型
     */
    int findKth(int n, int k) {
        //10叉树
        //https://blog.csdn.net/a1439775520/article/details/104913782
        int cur = 1;
        k--;

        while (k > 0) {
            long step = 0;
            long first = cur;
            long last = cur + 1;
            while (first <= n) {
                step += min(last, (long)(n + 1)) - first;
                first *= 10;
                last *= 10;
            }

            if (step > k) { //在树里
                k--;
                cur *= 10;
            }

            if (step <= k) { //不在树里
                k -= step;
                cur += 1;
            }
        }
        return cur;
    }
};