Implement basic calculator with four operations

Implement a basic arithmetic expression evaluator.

Problem

You are given a string s representing a mathematical expression containing non-negative integers and the operators '+', '-', '*', and '/'.

Compute and return the value of the expression.

Details & Assumptions

• The expression:
  • Contains only:
    • Non-negative integers in base 10 (may have multiple digits)
    • The operators '+' , '-' , '*' , '/'
    • Space characters ' ' (which should be ignored)
  • Is always syntactically valid (no malformed input, no division by zero).
• Operator precedence:
  • Multiplication '*' and division '/' have higher precedence than addition '+' and subtraction '-' .
  • Operators of the same precedence are evaluated from left to right (left-associative).
• Division is integer division that truncates toward zero .
• Parentheses do not appear in the input.

Function Signature (example)

You may implement a function in the language of your choice, for example:

int calculate(String s)

and return the evaluated integer result.

Examples

Example 1

• Input: "3+2*2"
• Output: 7
• Explanation: 2*2 = 4 , then 3 + 4 = 7 .

Example 2

• Input: " 14-3/2 "
• Output: 13
• Explanation: 3/2 truncates toward zero and becomes 1 , so 14 - 1 = 13 .

Example 3

• Input: "100/10*3+5"
• Output: 35
• Explanation: 100/10 = 10 , 10*3 = 30 , then 30 + 5 = 35 .

Constraints

• 1 <= s.length <= 100000
• The result always fits in a 32-bit signed integer.
• Aim for:
  • Time complexity: O(n) where n is the length of s .
  • Extra space: O(1) or O(n) (excluding the input string).