三元表达式解析

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 Frepresent 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 Tor 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"

string parseTernary(string expression) {
    // 三元运算是右结合，所以先处理最右的'?'，从右往左扫
    stack<char> stk;
    for (int i = (int)expression.size() - 1; i >= 0; i--) {
        char c = expression[i];
        if (c == '?') {
            auto expr1 = stk.top(); stk.pop();
            stk.pop(); // 略过:
            auto expr2 = stk.top(); stk.pop();
            stk.push(expression[i-1] == 'T' ? expr1 : expr2);
            i--; // 略过?前的T或F
        } else {
            stk.push(c);
        }
    }
    return string(1, stk.top());
}