寒假训练1.17训练赛E

题目
Polycarp loves ciphers. He has invented his own cipher called repeating.

Repeating cipher is used for strings. To encrypt the string s=s1s2…sm (1≤m≤10), Polycarp uses the following algorithm:

he writes down s1 ones,
he writes down s2 twice,
he writes down s3 three times,
…
he writes down sm m times.
For example, if s=“bab” the process is: “b” → “baa” → “baabbb”. So the encrypted s=“bab” is “baabbb”.

Given string t — the result of encryption of some string s. Your task is to decrypt it, i. e. find the string s.
题目大意
有一个字符串s=s1,s2,s3,…,sm(1<=m<=10)
小P将第一个字母写一次
将第二个字母写两次
将第三个字母写三次
将第m个字母写m次
形成一个新的字符串st。
现给出st,求s。
解题思路
s中的第k个字母为st中的第((1+k)*k))/2个字母
代码

#include <cstdio>
#include <string>
#include <iostream>
using namespace std;
int len;
int main()
{
	int len;
	string st;
	cin>>len;
	cin>>st;
	int num=0;
	int t=1;
	while (num<len)
	{
		cout<<st[num];
		num+=t;
		t++;
	}
	
}

来源：CSDN

作者：水墨青杉

链接：https://blog.csdn.net/weixin_45723759/article/details/104064522