Ternary Expression Parser — LeetCode 439 Python Solution

The problem

Given a string expression representing arbitrarily nested ternary expressions, evaluate the expression, and return the result of it. You can always assume that the given expression is valid and only contains digits, '?', ':', 'T', and 'F' where 'T' is true and 'F' is false.

Example

Input

expression = "T?2:3"

Output

"2"

Explanation

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

Python solution

class Solution:
    def parseTernary(self, expression: str) -> str:
        stk = []
        cond = False
        for c in expression[::-1]:
            if c == ':':
                continue
            if c == '?':
                cond = True
            else:
                if cond:
                    if c == 'T':
                        x = stk.pop()
                        stk.pop()
                        stk.append(x)
                    else:
                        stk.pop()
                    cond = False
                else:
                    stk.append(c)
        return stk[0]

Complexity

Pattern: Stack

When the most recent unresolved thing is the one that matters, use a stack. LeetCode 439. Ternary Expression Parser is filed here on both counts: the reference solution below belongs to the algorithm family this hub collects, and LeetCode tags it Stack.

The stack guide has the Python template for the pattern and the 194 LeetCode problems that use it.

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