LeetCode Ternary Expression Parser

原題連接在這裏：https://leetcode.com/problems/ternary-expression-parser/description/

題目：

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"

題解：

從後往前掃輸入的string. 把char 放入stack中. 當掃過了'?', 就知道須要開始判斷了.

從stack中彈出兩個數，判斷後的數放回stack中.

Time Complexity: O(n). n = expression.length();

Space: O(n).

AC Java:

 1 class Solution {
 2     public String parseTernary(String expression) {
 3         if(expression == null || expression.length() == 0){
 4             return expression;
 5         }
 6         
 7         Stack<Character> stk = new Stack<Character>();
 8         for(int i = expression.length()-1; i>=0; i--){
 9             char c = expression.charAt(i);
10             if(!stk.isEmpty() && stk.peek()=='?'){
11                 stk.pop(); // '?'
12                 char first = stk.pop();
13                 stk.pop(); // ':'
14                 char second = stk.pop();
15                 
16                 if(c == 'T'){
17                     stk.push(first);
18                 }else{
19                     stk.push(second);
20                 }
21             }else{
22                 stk.push(c);
23             }
24         }
25         
26         return String.valueOf(stk.peek());
27     }
28 }