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Balanced brackets in C++

It was a question on coursera course in Data structure and I spent some time on the program. Though I had taught the program to students earlier.

But somehow coursera is not accepting my submission:( So why not blog it?

The question is Write a program to balance brackets using stack. This is fundamental requirement in any programming language.

Let us look at some valid and invalid  strings.

{[]}()   valid
[]{}()[] valid
{([([])])} valid

{}) invalid
([)] invalid
({[]} invalid

You are getting the gist, right? When ever there is an opening bracket, there should be a corresponding closing bracket of matching type. And order is important, you can not have round bracket , square bracket followed by closing round bracket first and square bracket next.

So the algorithm is very simple
  1. create a stack of characters
  2. repeat for each i from 0 to length of string
    1. ch = ith character
    2. if ch is either { or [ or (, push it to stack
    3. if ch is closing bracket i.e. } or ] or } 
      1. get stack top charcter ch2
      2. if ch and ch2 are of same type pop the  char
      3. if not return a false
  3. if stack is empty all opening brackets are popped. Return true
  4. if stack is not empty, there are some extra opening brackets. Return false

Now for the code. I used the stack from stl in c++. Here is the code.


#include<stack>
#include<iostream>
using namespace std;
bool isBalanced(string str,char &wrongChar);
int main()
{
    
    string str;
    cout<<"Enter your string";
     getline(cin,str,'\n');
    char ch;
    bool bal = isBalanced(str,ch);
    if(bal)
      cout<<"success";
    else
      cout<<ch;
}

bool isBalanced(string str,char &wrongChar)
{
    stack<char> brackStack;
    int len = str.length();
    for(int i=0;i<len;i++)
        {
           char ch= str[i];
           if(ch=='(' || ch=='[' || ch=='{')
              brackStack.push(ch);
           else if ( ch==')' || ch==']' || ch=='}')
              {
                  char topChar = brackStack.top();
                  if(  (ch==')' && topChar=='(') ||
   (ch==']' && topChar=='[') ||
   (ch=='}' && topChar=='{') )
                   {
                      brackStack.pop();
                      continue;
                   }
                   else{
            /* not matching*/
            wrongChar = ch;
            return false;
     }
               }
         }
      if(brackStack.empty())
         return true;
      else
         {
           wrongChar = brackStack.top();
           return false;
          }
}

Not so tough? Isn't  it?

For more such questions and notes on all advanced topics, download my app c++ notes

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