224. Basic Calculator

Description

Implement a basic calculator to evaluate a simple expression string.

The expression string may contain open ( and closing parentheses ), the plus + or minus sign -, non-negative integers and empty spaces .

Note:

• You may assume that the given expression is always valid.

• Do not use the eval built-in library function.

Constraints

Approach

Links

• GeeksforGeeks

• Leetcode

• ProgramCreek

• YouTube

Examples

Example 1

Input: "1 + 1"

Output: 2

Example 2

Example 3

Solutions

Solution 1

/**
 * Time complexity : 
 * Space complexity : 
 */

class Solution {
    public int calculate(String s) {
        Stack<Integer> stack = new Stack<>();
        int n = s.length(), sign = 1, result = 0;
        
        for(int i = 0; i < n; i++) {
            if(Character.isDigit(s.charAt(i))) {
                int sum = s.charAt(i) - '0';
                while(i+1 < n && Character.isDigit(s.charAt(i+1))) {
                    sum = sum * 10 + (s.charAt(i+1) - '0');
                    i++;
                }
                result += sum * sign;
            } else if(s.charAt(i) == '+') {
                sign = 1;
            } else if(s.charAt(i) == '-') {
                sign = -1;
            } else if(s.charAt(i) == '(') {
                stack.push(result);
                stack.push(sign);
                result = 0;
                sign = 1;
            } else if(s.charAt(i) == ')') {
                result = result * stack.pop() + stack.pop();
            }
        }
        return result;
    }
}

Follow up