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Getting wrong answer for the question here is my link
http://codeforces.com/contest/797/problem/C

What I have tried:

#include<bits/stdc++.h>    
      using namespace std; 
      int main(){    
    string s;    
    int i,j,k,n;    
    cin>>s;    
    string u="";    
    vector<int>hash(27,0);     
      stack<char>st;    
        i=0;    
    for(i=0;i<s.size();i++)    
          hash[s[i]-'a'+1]++;    
    i=0;    
    int check;    
    while(i<s.size()){    
     check=0;    
    for(j=0;j<s[i]-'a'+1;j++)     
            {         
         if(hash[j]!=0) {check=1;break;}  
     }      
    if(check==1)   
          {     st.push(s[i]);    
              hash[s[i]-'a'+1]--;    
                         i++;     
           }    
     else {u+=s[i];  hash[s[i]-'a'+1]--;i++;}    
    }     
    while(!st.empty())    
          {           
              u+=st.top();     
               st.pop();      
    }    
    cout<<u;          
    return 0; 
    }
Posted
Updated 15-Dec-17 23:50pm

Do you want a list?
For starters, indent it correctly - that is all over the place and that makes it very hard to read and work out what goes where.
Then, stop using single character variable names, and use names which are relevant to teh task they are doing. It's a little more typing, but it saves a huge amount of time in teh long run as the code becomes more readable.

Then ... this is a "contest" which means it's effectively homework, and we do not do your homework: it is set for a reason. It is there so that you think about what you have been told, and try to understand it.

And finally ... Compiling does not mean your code is right! :laugh:
Think of the development process as writing an email: compiling successfully means that you wrote the email in the right language - English, rather than German for example - not that the email contained the message you wanted to send.

So now you enter the second stage of development (in reality it's the fourth or fifth, but you'll come to the earlier stages later): Testing and Debugging.

Start by looking at what it does do, and how that differs from what you wanted. This is important, because it give you information as to why it's doing it. For example, if a program is intended to let the user enter a number and it doubles it and prints the answer, then if the input / output was like this:
Input   Expected output    Actual output
  1            2                 1
  2            4                 4
  3            6                 9
  4            8                16
Then it's fairly obvious that the problem is with the bit which doubles it - it's not adding itself to itself, or multiplying it by 2, it's multiplying it by itself and returning the square of the input.
So with that, you can look at the code and it's obvious that it's somewhere here:
int Double(int value)
   {
   return value * value;
   }

Once you have an idea what might be going wrong, start using the debugger to find out why. Put a breakpoint on your line:
i=0;

and run your app. Think about what each line in the code should do before you execute it, and compare that to what it actually did when you use the "Step over" button to execute each line in turn. Did it do what you expect? If so, move on to the next line.
If not, why not? How does it differ?

This is a skill, and it's one which is well worth developing as it helps you in the real world as well as in development. And like all skills, it only improves by use!
   
v2
Learn to indent properly your code, it show its structure and it helps reading and understanding. It also helps spotting structures mistakes.
#include<bits/stdc++.h>
using namespace std;
int main(){
  string s;
  int i,j,k,n;
  cin>>s;
  string u="";
  vector<int>hash(27,0);
  stack<char>st;
  i=0; // not necessary
  for(i=0;i<s.size();i++)
    hash[s[i]-'a'+1]++;
  i=0;
  int check;
  while(i<s.size()){
    check=0;
    for(j=0;j<s[i]-'a'+1;j++)
    {
      if(hash[j]!=0) {
        check=1;
        break;
      }
    }
    if(check==1)
    {
      st.push(s[i]);
      hash[s[i]-'a'+1]--;
      i++;
    }
    else {
      u+=s[i];
      hash[s[i]-'a'+1]--;
      i++;
    }
  }
  while(!st.empty())
  {
    u+=st.top();
    st.pop();
  }
  cout<<u;
  return 0;
}

Professional programmer's editors have this feature and others ones such as parenthesis matching and syntax highlighting.
Notepad++ Home[^]
ultraedit[^]

Advices:
- Do not pack code in single line, it makes things just more difficult to read with no gain. See indented code.
- Adopt single style of writing with { and }, it makes code easier to read.
- Comment your code, it help others to understand what you do.
Quote:
What is wrong with my code?
Getting wrong answer for the question

- Try all sample inputs, do you get the right answers?
- Invent some bigger inputs that are more tricky, do you get the right answers?
When answer is wrong, is it your algorithm or its translation to code ?
   

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