Program to evaluate ternary expression in C++

Suppose we have an expression that holds the ternary expression, we have to evaluate the result of the expression. It supports some values like T and F for True and False and “?” and “:” characters. There are some properties:

• The length of the given string must be less than or equal to 10000.
• The conditional expressions group right-to-left.
• The condition will always be either T or F. So the condition will never be a digit.
• The result of the expression will always evaluate to T or F.

So for example, if the input is like “T ? T ? F : T : T”, so the output will be F.

To solve this, we will follow these steps:

• ret := an empty string, n := size of s,
• create a stack st
• for i in range n – 1 down to 0
  • x := s[i]
  • if st is not empty and top of stack is ‘?’, then
    • delete from st
    • first := top of st, then delete two elements from stack
    • second := top of stack, and delete from st
    • if x is T, then insert first into st, otherwise insert second into st
  • otherwise insert x into st
• while st is not empty, then
  • ret := ret + top of st and delete from st
• reverse ret and return ret

Let us see the following implementation to get better understanding:

Example

#include
using namespace std;
class Solution {
   public:
   string parseTernary(string s) {
      string ret = "";
      int n = s.size();
      stack st;
      for(int i = n - 1; i >= 0; i--){
         char x = s[i];
         if(!st.empty() && st.top() == '?'){
            st.pop();
            char first = st.top();
            st.pop();
            st.pop();
            char second = st.top();
            st.pop();
            if(x == 'T'){
               st.push(first);
            }
            else st.push(second);
         }
         else{
            st.push(x);
         }
      }
      while(!st.empty()){
         ret += st.top();
         st.pop();
      }
      reverse(ret.begin(), ret.end());
   return ret;
   }
};
main(){
   Solution ob;
   cout << (ob.parseTernary("T?T?F:T:T"));
}

Input

" T?T?F:T:T"

Output

F