站外题求hack or debug

You’re typing a long text with a broken keyboard. Well it’s not so badly broken. The only problem
 with the keyboard is that sometimes the “home” key or the “end” key gets automatically pressed
 (internally).
 You’re not aware of this issue, since you’re focusing on the text and did not even turn on the
 monitor! After you finished typing, you can see a text on the screen (if you turn on the monitor).
 In Chinese, we can call it Beiju. Your task is to find the Beiju text.
 Input
 There are several test cases. Each test case is a single line containing at least one and at most 100,000
 letters, underscores and two special characters ‘[’ and ‘]’. ‘[’ means the “Home” key is pressed
 internally, and ‘]’ means the “End” key is pressed internally. The input is terminated by end-of-file
 (EOF).
 Output
 For each case, print the Beiju text on the screen.
 Sample Input
 This_is_a_[Beiju]_text
 [[]][][]Happy_Birthday_to_Tsinghua_University
 Sample Output
 BeijuThis_is_a__text
 Happy_Birthday_to_Tsinghua_University

#include<bits/stdc++.h>
using namespace std;
string str;
char arr[100050];
int nxt[100050];
int main()
{
	while(cin>>str)
	{
		int len=str.length();
		int ptr=0;
		int last=0;
		int blank=2;
		nxt[0]=1;
		for(int i=0;i<len;i++)
		{
			if(str[i]=='[')
			{
				ptr=0;
			}
			else if(str[i]==']')
			{
				ptr=last;
			}
			else
			{
				nxt[blank]=nxt[ptr];
				nxt[ptr]=blank;
				arr[blank]=str[i];
				blank++;
				if(ptr==last)
				{
					last=nxt[ptr];
				}
				ptr=nxt[ptr];
			}
		}
		int tmp=0;
		while(tmp!=1)
		{
			cout<<arr[nxt[tmp]];
			tmp=nxt[tmp];
		}
		cout<<endl;
	}
	return 0;
}