Ternary Expression Parser Leetcode

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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.

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"

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"

public class Solution {
    public String parseTernary(String expression) {
        Stack<Character> s = new Stack<>();
        char[] str = expression.toCharArray();
        for (int i = str.length - 1; i >= 0; i--) {
            if (str[i] == ‘?‘) {
                char first = s.pop();
                s.pop();
                char second = s.pop();
                if (str[i - 1] == ‘T‘) {
                    s.push(first);
                } else {
                    s.push(second);
                }
                i--;
            } else {
                s.push(str[i]);
            }
        }
        return String.valueOf(s.pop());
    }
}

Ternary Expression Parser Leetcode

标签：else   content   ril   character   cal   int   bit   note   example   

原文地址：http://www.cnblogs.com/aprilyang/p/6414825.html