224. Basic Calculator

Description

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 * 105
• 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.

 

Solutions

Solution: Stack

• Time complexity: O(n)
• Space complexity: O(n)

 

JavaScript

/**
 * @param {string} s
 * @return {number}
 */
const calculate = function (s) {
  const signStack = [1];
  let sign = 1;
  let result = 0;

  for (let index = 0; index < s.length; index++) {
    let current = s[index];

    if (current === ' ') continue;
    if (current === '(') {
      signStack.push(signStack.at(-1) * sign);
      sign = 1;
    } else if (current === ')') signStack.pop();
    else if (current === '+') sign = 1;
    else if (current === '-') sign = -1;
    else {
      while (/\d/.test(s[index + 1])) {
        index += 1;
        current += s[index];
      }
      result += sign * current * signStack.at(-1);
    }
  }
  return result;
};