772. Basic Calculator III

Solve on LintCode

Description

Implement a basic calculator to evaluate a simple expression string.

The expression string contains only non-negative integers, '+', '-', '*', '/' operators, and open '(' and closing parentheses ')'. The integer division should truncate toward zero.

You may assume that the given expression is always valid. All intermediate results will be in the range of [-231, 231 - 1].

Note: You are not allowed to use any built-in function which evaluates strings as mathematical expressions, such as eval().

 

Example 1:

Input: s = "1+1"
Output: 2

Example 2:

Input: s = "6-4/2"
Output: 4

Example 3:

Input: s = "2*(5+5*2)/3+(6/2+8)"
Output: 21

 

Constraints:

  • 1 <= s <= 104
  • s consists of digits, '+', '-', '*', '/', '(', and ')'.
  • s is a valid expression.

Solutions

Read: Solving Basic Calculator I, II, III on leetcode

Approach 1

SAME as Basic Calculator I: Approach 1

Python Code
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import collections
class Solution:
    def calculate(self, s: str) -> int:
        def helper(q):
            stack = []
            sign = '+'
            num = 0
            while q:
                c = q.popleft()
                if c.isdigit():
                    num = num*10 + int(c)
                if c == '(':
                    num = helper(q)
                if c in '+-*/)' or not q:
                    if sign == '+':
                        stack.append(num)
                    elif sign == '-':
                        stack.append(-num)
                    elif sign == '*':
                        stack.append(stack.pop() * num)
                    elif sign == '/':
                        stack.append(int(stack.pop()/num))
                    sign = c
                    num = 0
                    if c == ')':
                        break
            return sum(stack)
                    

        q = collections.deque()
        for c in s:
            q.append(c)
        return helper(q)

Time: $O(n)$ Space: $O(n)$