fix: operator precedence for leetcode 227 [basic calculator ii]

c++/basic-calculator-ii.md

@@ -31,25 +31,30 @@ int calculate(std::string s) {
     std::stack<char> operators;
     int n = s.length();
     int currentNumber = 0;
-    
+
     for (int i = 0; i < n; i++) {
         char c = s[i];
-
         // If the current character is a digit, form the whole number
         if (isdigit(c)) {
             currentNumber = currentNumber * 10 + (c - '0');
         }
-
-        // If the current character is an operator, or it's the last character
+
+        // If the current character is an operator, or it's the last
+        // character
         if (!isdigit(c) && !isspace(c) || i == n - 1) {
             if (!operators.empty()) {
-                // If the previous operator was '*' or '/', perform that operation
                 char prevOp = operators.top();
                 if (prevOp == '*' || prevOp == '/') {
                     operators.pop();
                     int prevNumber = numbers.top();
                     numbers.pop();
-                    currentNumber = (prevOp == '*') ? prevNumber * currentNumber : prevNumber / currentNumber;
+                    currentNumber = (prevOp == '*')
+                                        ? prevNumber * currentNumber
+                                        : prevNumber / currentNumber;
+                } else if (prevOp == '-') {
+                    operators.pop();
+                    operators.push('+');
+                    currentNumber = -currentNumber;
                 }
             }
             numbers.push(currentNumber);
@@ -59,22 +64,25 @@ int calculate(std::string s) {
             }
         }
     }
-
-    // Now process remaining operators for addition or subtraction
-    int result = numbers.top();
-    numbers.pop();
+
+    if (operators.size() == 0)
+        return numbers.top();
+    int result = 0;
+    if (currentNumber != 0)
+        result = currentNumber;
+    else {
+        result = numbers.top();
+        numbers.pop();
+    }
+
     while (!numbers.empty()) {
+        int first = numbers.top();
+        numbers.pop();
         char op = operators.top();
         operators.pop();
-        int num = numbers.top();
-        numbers.pop();
-        if (op == '+') {
-            result += num;
-        } else if (op == '-') {
-            result = num - result;
-        }
+        result = (result + first);
     }
-    
+
     return result;
 }
 
@@ -87,7 +95,7 @@ int main() {
 
 #### Complexity:
 - **Time Complexity:** O(n), where n is the length of the string, since we are iterating once through the input.
-- **Space Complexity:** O(n), due to the supplementary space used by two stacks.
+- **Space Complexity:** O(n), due to the supplementary space used by four stacks.
 
 ---