224-BasicCalculator

Basic Calculator

Hard

Given a string s representing a valid expression, implement a basic calculator to evaluate it, and return the result of the evaluation.

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 = " 2-1 + 2 "
Output: 3

Example 3:

Input: s = "(1+(4+5+2)-3)+(6+8)"
Output: 23

Constraints:

• $1$ <= s.length <= $3 * 10^5$
• s consists of digits, '+', '-', '(', ')', and ' '.
• s represents a valid expression.
• '+' is not used as a unary operation (i.e., "+1" and "+(2 + 3)" is invalid).
• '-' could be used as a unary operation (i.e., "-1" and "-(2 + 3)" is valid).
• There will be no two consecutive operators in the input.
• Every number and running calculation will fit in a signed 32-bit integer.

Problem can be found in here!

Solution

Solutions below can be used to solve Basic Calculator, Basic Calculator II, and Basic Calculator III.

Solution1: Stack + Recursion

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

Solution2: Postfix

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