impl basic calculator II

leetcode-rs/src/solutions/basic_calculator_ii.rs

@@ -13,6 +13,27 @@ crate::derive_solution!(
 
 impl SolutionImpl {
     pub fn calculate(s: String) -> i32 {
-        todo!()
+        let mut stack = Vec::new();
+        let mut prev_sign = '+';
+        let mut num = 0;
+
+        for (idx, ch) in s.chars().enumerate() {
+            if ch.is_ascii_digit() {
+                num = num * 10 + ch.to_digit(10).unwrap() as i32;
+            }
+            if !ch.is_ascii_digit() && ch != ' ' || idx == s.len() - 1 {
+                match prev_sign {
+                    '+' => stack.push(num),
+                    '-' => stack.push(-num),
+                    '*' => *stack.last_mut().unwrap() *= num,
+                    '/' => *stack.last_mut().unwrap() /= num,
+                    _ => {}
+                }
+                num = 0;
+                prev_sign = ch;
+            }
+        }
+
+        stack.iter().sum()
     }
 }