From 70984609e9df26d492878fd90babf9bd85417b1f Mon Sep 17 00:00:00 2001
From: "Gumparthy Pavan Kumar[pk]" <gumparthypk@gmail.com>
Date: Sun, 27 Jul 2025 10:49:03 +0530
Subject: [PATCH] fix: operator precedence for leetcode 227 [basic calculator
 ii]
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1. What is now different from before?
> The implementation now reverses the numbers and operators stacks before applying + and - operators.This ensures that + and - are evaluated in left-to-right order, which matches the expected behavior of the expression (i.e., left-associative evaluation). Previously, operators were applied in reverse order, leading to incorrect results like 1-1+1 → -1 instead of 1.
2. What is the reason for the change?
> + and - are left-associative operators with the same precedence, and must be processed in the order they appear.
3. Anything to watch out for?
> Space complexity increases slightly due to the temporary numRev and opRev stacks, but remains O(n).
---
 c++/basic-calculator-ii.md | 31 ++++++++++++++++++++++---------
 1 file changed, 22 insertions(+), 9 deletions(-)

diff --git a/c++/basic-calculator-ii.md b/c++/basic-calculator-ii.md
index 0c76c1d..e6b978f 100644
--- a/c++/basic-calculator-ii.md
+++ b/c++/basic-calculator-ii.md
@@ -60,18 +60,31 @@ int calculate(std::string s) {
         }
     }
     
-    // Now process remaining operators for addition or subtraction
-    int result = numbers.top();
-    numbers.pop();
-    while (!numbers.empty()) {
-        char op = operators.top();
-        operators.pop();
-        int num = numbers.top();
+    // Reverse the stacks to maintain left-to-right order
+    std::stack<int> numbersRev;
+    std::stack<int> operatorsRev;
+    while(!numbers.empty()) {
+        numbersRev.push(numbers.top());
         numbers.pop();
+    }
+
+    while(!operators.empty()) {
+        operatorsRev.push(operators.top());
+        operators.pop();
+    }
+
+    // Now process remaining operators for addition or subtraction
+    int result = numbersRev.top();
+    numbersRev.pop();
+    while (!numbersRev.empty()) {
+        char op = operatorsRev.top();
+        operatorsRev.pop();
+        int num = numbersRev.top();
+        numbersRev.pop();
         if (op == '+') {
             result += num;
         } else if (op == '-') {
-            result = num - result;
+            result -= num;
         }
     }
     
@@ -87,7 +100,7 @@ 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.
 
 ---