checking the validity of a string expression according to some rules

Question

I have this tricky problem that I couldn't solve :

we have a string that contains either "{}" or "[]" sequences, I want to write a function or method that checks if a string is valid: It should retrun true :

if a string is empty
if E is valid (containing only the sequences above) then {E} or [E] is valid too.

or if E and F are concatenantion of valid expressions, the result is also valid.

"[{}]" // valid
"{[}]" // invalid
"{{[[]]}}" // valid
"{{[[]]}}{{[[]]}}" // valid

I tried to solve that problem by scanning the string char by char, I didn't find the suitable algorithm then I thought about regex_match, and finally I said to myself that this kind of problem should be solved with a state machine (like EBNF).

What can you do to solve that problem ?

This a traditional "introduction to the stack data structure" problem. — molbdnilo, Mar 02 '17 at 11:25
Just by simple googling [here](http://www.geeksforgeeks.org/check-for-balanced-parentheses-in-an-expression/) — Anand Undavia, Mar 02 '17 at 11:29
I can check the equality of the first char and the last "n" then the second and "n-1" etc... that should fix the second point but it won't work for concatenated strings. — Aminos, Mar 02 '17 at 11:30
@Aminos: As in, you have source code for that? Would be useful to edit it into your question, as it can be a building block. — MSalters, Mar 02 '17 at 11:32
Thank you all, I ll answer my question after studying the link Anand gave me. Have a good day. — Aminos, Mar 02 '17 at 11:34
Hint: use a stack, when encountering a opening push it, when encountering a closing pop element on the stack and verify correspondance, at the end you should have read everything and the stack should be empty. — Jean-Baptiste Yunès, Mar 02 '17 at 11:54
@PaulHankin I took a code from the question you gave me and I converted it to C++. Thank you. — Aminos, Mar 02 '17 at 17:30

score 0 · Accepted Answer · answered Mar 02 '17 at 17:36

Simple answer in C++

   static bool check(const string& sequence)
   {
      if (sequence.empty())
         return true;

      std::stack<char> stack;

      for (size_t i = 0; i < sequence.length(); ++i)
      {
         const char current = sequence.at(i);
         if (current == '{' || current == '[')
            stack.push(current);

         if (current == '}' || current == ']')
         {
            if (stack.empty())
               return false;

            char last = stack.top();
            if (current == '}' && last == '{' || current == ']' && last == '[')
               stack.pop();
            else
               return false;
         }
      }

      return stack.empty();
   }

checking the validity of a string expression according to some rules

1 Answers1