439. Ternary Expression Parser

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 digits 0-9, ?, :, T and F (T and F represent 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"

// Two Pass + Stack
string eval(const string& str) {
    if (str.length() != 5) return "";
    return str[0] == 'T' ? str.substr(2, 1) : str.substr(4);
}
string parseTernary(string expression) { // time: O(n); space: O(n)
    string res = expression;
    stack<int> st; // store positions of '?'
    for (int i = 0; i < expression.length(); ++i) {
        if (expression[i] == '?') st.push(i);
    }
    while (!st.empty()) {
        int t = st.top(); st.pop();
        res = res.substr(0, t - 1) + eval(res.substr(t - 1, 5)) + res.substr(t + 4);
    }
    return res;
}

// Iteration with Stack
string parseTernary(string expression) { // time: O(n); space: O(n)
    stack<char> st;
    for (int i = expression.length() - 1; i >= 0; --i) {
        char c = expression[i];
        if (!st.empty() && st.top() == '?') {
            st.pop();
            char first = st.top(); st.pop();
            st.pop(); // skip ':'
            char second = st.top(); st.pop();
            st.push(c == 'T' ? first : second);
        } else {
            st.push(c);
        }
    }
    return string(1, st.top());
}

// Bottom-Up Recursion
char helper(const string& expression, int& idx) { 
    char ch = expression[idx];
    if (idx == expression.length() - 1 || expression[idx + 1] == ':') {
        idx += 2; // skip ':'
        return ch;
    }
    idx += 2; // skip '?'
    char first = helper(expression, idx), second = helper(expression, idx);
    return ch == 'T' ? first : second;
}
string parseTernary(string expression) { // time: O(n); space: O(n)
    int idx = 0;
    return string(1, helper(expression, idx));
}