Given a string representing arbitrarily nested ternary expressions, calculate the result of the expression. You can always assume that the given expression is valid and only consists of digits0-9,?,:,TandF(TandFrepresent True and False respectively).

Note:

1. The length of the given string is ≤ 10000.
2. Each number will contain only one digit.
3. The conditional expressions group right-to-left (as usual in most languages).
4. The condition will always be either T or F . That is, the condition will never be a digit.
5. The result of the expression will always evaluate to either a digit 0-9 , T or F .

Example 1:

Input:
 "T?2:3"


Output:
 "2"


Explanation:
 If true, then result is 2; otherwise result is 3.

Example 2:

Input:
 "F?1:T?4:5"


Output:
 "4"


Explanation:
 The conditional expressions group right-to-left. Using parenthesis, it is read/evaluated as:

             "(F ? 1 : (T ? 4 : 5))"                   "(F ? 1 : (T ? 4 : 5))"
          -
>
 "(F ? 1 : 4)"                 or       -
>
 "(T ? 4 : 5)"
          -
>
 "4"                                    -
>
 "4"

Example 3:

Input:
 "T?T?F:5:3"


Output:
 "F"


Explanation:
 The conditional expressions group right-to-left. Using parenthesis, it is read/evaluated as:

             "(T ? (T ? F : 5) : 3)"                   "(T ? (T ? F : 5) : 3)"
          -
>
 "(T ? F : 3)"                 or       -
>
 "(T ? F : 5)"
          -
>
 "F"                                    -
>
 "F"

class Solution {
    public String parseTernary(String expression) {
        if (expression == null || expression.length() == 0) return expression;
        int index = expression.length() - 1;
        Stack<Character> stack = new Stack<>();

        while (index >= 0){
            char cur = expression.charAt(index);
            if (cur == '?' && !stack.isEmpty()){
                char first = expression.charAt(index - 1);
                char second = stack.pop();
                char third = stack.pop();

                char res = first == 'T' ? second : third;

                stack.push(res);
                index = index - 2;
            }
            else if (cur == ':'){
                index--;
            }
            else{
                stack.push(cur);
                index--;
            }
        }

        return stack.pop() + "";
    }
}