Describe a PDA that accepts all strings over $\{a, b\}$ that have as many $a$’s as $b$’s. I'm having my exam in few days and I would like help with this 

Describe a PDA that accepts all strings over $\{ a, b \}$ that have as many $a$’s as $b$’s.


Hint: Use the stack as an indication of how many more of one symbol have been so far read from the string than the other.  (Also ensure that the stack never contains both $\mathtt{a}$s and $\mathtt{b}$s at the same time.)
