Can someone plz tell me why i am getting WA ! which test case i am missing ?

https://www.codechef.com/viewsolution/26554850

How my solution is wrong ?

```
> #include<iostream>
> #include<stack>
> #include<vector>
> using namespace std;
>
> int main()
> {
> int t;
> vector<int> ans;
> cin>>t;
> while(t--)
> {
> stack<char> expr;
> string str;
> cin>>str;
> int sum=0;
> for(int i=0;i<str.length();i++)
> {
> if(str[i]=='<')
> expr.push(str[i]);
> else if(!expr.empty())
> {
> expr.pop();
> sum+=2;
> }
> else
> break;
> }
> ans.push_back(sum);
> }
> for(auto i=ans.begin();i!=ans.end();i++)
> cout<<*i<<"\n";
> return 0;
> }
>
```

The answer is correct. It should be 0, since there are no "lapin"s at the prefix.

Can anyone tell what’s wrong with this code?

```
#include <iostream>
#include <vector>
#include <stack>
using namespace std;
#define ull unsigned long long int
int getK(vector<char> s) {
stack<char> st;
int k=0;
for(int i=0; i<s.size(); i++) {
if(s[i]=='<') {
st.push('<');
}
else { // '>'
if(!st.empty()) {
st.pop();
k++;
}
else {
break;
}
}
}
return k;
}
int main() {
int T;
cin >> T;
while(T--) {
string ss;
cin >> ss;
vector<char> s(ss.begin(),ss.end());
int K = getK(s);
if(K==getK(vector<char>(ss.begin()+1,ss.end()))) cout << 0 << endl;
else cout << K*2 << endl;
}
}
```

can anyone plz say why this is giving WA ?? Any particular case

Consider the testcase:

```
1
<<>
```

Kindly check why this is giving WA.

**My logic** : If ‘<’ increase top pointer. If ‘>’ decrease top pointer and increase length by 2 (1 for < and 1 for >)

Initial top pointer value = -1. If ‘>’ and top pointer is -1 it means no matching ‘<’ encountered…so return the current length of valid string till now.

Your solution fails on the testcase in the post above yours

Yours is the only solution that I understood properly. Thanks for sharing!

I tried solving this problem with implementing concept of stack but it is giving WA. Although in my PC’s IDE it worked well.

Please once check my solution(https://www.codechef.com/viewsolution/31943162) it’s small.

Thank you so much sir.

Thanks. I saw your post but forgot to reply. today i just happened to pass by and noticed it

I think <<> should give 0 ?

Yep

Solution adjusted accordingly…but still getting WA for some test case…Can you visualise any such case?

https://www.codechef.com/viewsolution/32322530.

MW it passed the <<> case

///maybe this is what you asked

#include <bits/stdc++.h>

#include

#include

#define ll long long int

using namespace std;

bool func(string s,int i,int j)

{

```
stack<char> stk;
for(;i<=j;i++)
{
if(s[i]=='<')
stk.push(s[i]);
else
{ if(stk.empty()||stk.top()!='<')
return false;
else
stk.pop();
}
}
return stk.empty();
```

}

int main() {

ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);

int t;cin>>t;

while(t–)

{

string s;

cin>>s;

stack stk;

vector v;

```
for(int i=0;i<s.length();i++)
{
if(s[i]=='<')
{
for(int j=i+1;j<s.length();j++)
{
if(s[j]=='>')
{
if(func(s,i,j)==true)////if(func(s,i+1,j-1)==true)
v.push_back(j-i+1);
}
}
}
}
if(v.size()==0)
cout<<"0\n";
else
cout<<*max_element(v.begin(),v.end())<<"\n";
}
```

}

Can anyone please explain what is longest prefix length means it’s confusing please break it in simple terms.

Thanks