# 0772. Basic Calculator III

<https://leetcode.com/problems/basic-calculator-iii>

## Description

Implement a basic calculator to evaluate a simple expression string.

The expression string contains only non-negative integers, `'+'`, `'-'`, `'*'`, `'/'` operators, and open `'('` and closing parentheses `')'`. 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()`.

**Example 1:**

```
**Input:** s = "1+1"
**Output:** 2
```

**Example 2:**

```
**Input:** s = "6-4/2"
**Output:** 4
```

**Example 3:**

```
**Input:** s = "2*(5+5*2)/3+(6/2+8)"
**Output:** 21
```

**Example 4:**

```
**Input:** s = "(2+6*3+5-(3*14/7+2)*5)+3"
**Output:** -12
```

**Example 5:**

```
**Input:** s = "0"
**Output:** 0
```

**Constraints:**

* `1 <= s <= 104`
* `s` consists of digits, `'+'`, `'-'`, `'*'`, `'/'`, `'('`, and `')'`.
* `s` is a **valid** expression.

## ac: Stack

```java
class Solution {
    public int calculate(String s) {
        // Edge cases
        if (s == null || s.length() == 0) return 0;
        
        Deque<Character> operators = new ArrayDeque<>();
        Deque<Integer> nums = new ArrayDeque<>();
        
        int currNum = 0;
        for (int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);
            if (c == ' ') continue;
            if (Character.isDigit(c)) {
                currNum = currNum * 10 + (c - '0');
            } else if (c == '+' || c == '-' || c == '*' || c == '/') {
                while (!operators.isEmpty() && canDoPreviousOperation(operators.peek(), c)) {
                    currNum = doOperation(nums.pop(), operators.pop(), currNum);
                }
                
                operators.push(c);
                nums.push(currNum);
                currNum = 0;
            } else if (c == '(') {
                operators.push('(');
            } else if (c == ')') {
                while (operators.peek() != '(') {
                    currNum = doOperation(nums.pop(), operators.pop(), currNum);
                }
                operators.pop(); // evict '('
            }
        }
        
        while (!operators.isEmpty()) {
            currNum = doOperation(nums.pop(), operators.pop(), currNum);
        }
        
        return currNum;
    }
    
    private int doOperation(int num1, Character operator, int num2) {
        switch (operator) {
            case '+': return num1 + num2;
            case '-': return num1 - num2;
            case '*': return num1 * num2;
            case '/': return num1 / num2;
            default:
                throw new IllegalArgumentException("Invalid operator.");
        }
    }
    
    private boolean canDoPreviousOperation(Character op1, Character op2) {
        if (op1 == '(') return false;
        if (op2 == '+' || op2 == '-') {
            return true;
        } else {
            // op2 is * or /
            if (op1 == '*' || op1 == '/') return true; 
            
            return false;
        }
        
    }
}
/*
E.g. ...(6 - 4 / 2)...
num1, op1, currNum, {currOp}, {num3} ...
1. currOp is +/-, or op1 && currOp both are * or /: currNum = eval(num1, op1, currNum)
2. other cases: currNum = eval(currNum, currOp, num3). Because we don't know whether it is left parenthesis or num3 after currOp, let's push currNum, currOp to stack and do calculation later.
stack operators: [-]
stack nums: [6]
*/
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://jaywin.gitbook.io/leetcode/solutions/0772-basic-calculator-iii.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.