3.8——K-th Substring

题目描述

You are given a string s. Among the different substrings of s, print the K-th lexicographically smallest one.
A substring of s is a string obtained by taking out a non-empty contiguous part in s. For example, if s = ababc, a, bab and ababc are substrings of s, while ac, z and an empty string are not. Also, we say that substrings are different when they are different as strings.
Let X=x1x2…xn and Y=y1y2…ym be two distinct strings. X is lexicographically larger than Y if and only if Y is a prefix of X or xj>yj where j is the smallest integer such that xj≠yj.

Constraints
1 ≤ |s| ≤ 5000
s consists of lowercase English letters.
1 ≤ K ≤ 5
s has at least K different substrings.
Partial Score
200 points will be awarded as a partial score for passing the test set satisfying |s| ≤ 50.

输入

Input is given from Standard Input in the following format:
s
K

输出

Print the K-th lexicographically smallest substring of K.

样例输入

复制样例数据

aba
4

样例输出

b

提示

s has five substrings: a, b, ab, ba and aba. Among them, we should print the fourth smallest one, b. Note that we do not count a twice.

来源/分类

ABC097&ARC097 

意思就是找字串，求出第k大的。

#include <cstdio>
#include <iostream>
#include <cstring>
#include <set>
using namespace std;

set<string> p;
int main ()
{
    string s;
    int k;
    cin>>s>>k;
    int i,j;
    int n=s.size();
    for (i=0;i<=n-1;i++)
        for (j=1;j<=5;j++)
        {
            if (i+j-1>=n) continue;
            p.insert(s.substr(i,j));
        }
    for (i=1;i<=k-1;i++)
    {
        p.erase(p.begin());
    }
    cout<<(*(p.begin()))<<endl;//这样使用
    return 0;
}