Basic calculator ii

Math String Stack

46.7% acceptance

Jan 12, 2026

6584

950

Given a string s which represents an expression, evaluate this expression and return its value. 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().

Solution

Rust

Time O(n)

Space O(n)

LeetCode

solution.rs

impl Solution {
  pub fn calculate(s: String) -> i32 {
    let mut stack = vec![];
    let mut num = 0;
    let mut op = '+';
    
    for (i, ch) in s.chars().enumerate() {
      if ch.is_ascii_digit() {
        num = num * 10 + (ch as i32 - '0' as i32);
      }
      if (!ch.is_ascii_digit() && ch != ' ') || i == s.len() - 1 {
        match op {
          '+' => stack.push(num),
          '-' => stack.push(-num),
          '*' => {
            let last = stack.pop().unwrap();
            stack.push(last * num);
          }
          '/' => {
            let last = stack.pop().unwrap();
            stack.push(last / num);
          }
          _ => {}
        }
        op = ch;
        num = 0;
      }
    }
    stack.iter().sum()
  }
}