[leetcode 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: 2Output:10Explanation:The lexicographical order is [1, 10, 11, 12, 13, 2, 3, 4, 5, 6, 7, 8, 9], so the second smallest number is 10.

给定数n，然后1-n按照字典排序，找出第k个数字

首先求（n,n+1）按照字典排序之间包含的数量sum

如果sum > k : n*=10 then k--

如果sum <= K:n++ then k-=sum

AC代码：

public class Solution {     public static int findKthNumber(int n, int k) {        int res = 1;        --k;        while(k > 0){            long sum = getSum(res, n);            if(sum <= k){                k -=sum;                ++res;            }else{                res *= 10;                --k;            }        }        return res;    }    public static long getSum(long start,long n){        long end = start + 1;        long sum = 0;        while(start <= n){            sum += Math.max(0,(Math.min(n+1,end)-start));            start *=10;            end *= 10;        }        return sum ;    }}

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