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224. Basic Calculator

Problem Description

Implement a basic calculator to evaluate a simple expression string.

The expression string may contain open ( and closing parentheses ), the plus + or minus sign -, non-negative integers and empty spaces .

Example 1:

Input: "1 + 1"
Output: 2

Example 2:

Input: " 2-1 + 2 "
Output: 3

Example 3:

Input: "(1+(4+5+2)-3)+(6+8)"
Output: 23

Solution

Approach 1: One Stack

// TODO

Approach 2: Two Stack

geeksforgeeks tutorial

public int calculate(String s) {
    int[] precedence = new int[128];
    precedence['+'] = 1; precedence['-'] = 1;
    precedence['*'] = 2; precedence['/'] = 2;
    
    Stack<Integer> vals = new Stack<>();
    Stack<Character> ops = new Stack<>();
    System.out.println(Arrays.toString(split(s)));
    
    for (String token : split(s)) {
        char c = token.charAt(token.length() - 1);
        if (Character.isDigit(c))
            vals.push(Integer.parseInt(token));
        else if (c == '(')
            ops.push(c);
        else if (c ==  ')') {
            while (!ops.isEmpty() && ops.peek() != '(') {
                int num2 = vals.pop();
                int num1 = vals.pop();
                int result = operate(num1, num2, ops.pop());
                vals.push(result);
            }
            ops.pop();
        } else {
            while (!ops.isEmpty() && precedence[ops.peek()] >= precedence[c]) {
                int num2 = vals.pop();
                int num1 = vals.pop();
                int result = operate(num1, num2, ops.pop());
                vals.push(result);
            }
            ops.push(c);
        }
    }
    while (!ops.isEmpty()) {
        int num2 = vals.pop();
        int num1 = vals.pop();
        int result = operate(num1, num2, ops.pop());
        vals.push(result);
    }
    
    return vals.pop();
}

private helper methods

private int operate(int a, int b, char op){ 
    switch(op) {
        case '+': return a + b; 
        case '-': return a - b; 
        case '*': return a * b; 
        case '/': return a / b; 
    }
    return -1;
}

// not efficient but easy to understand
private String[] split(String s) {        
    String[] chars = {"+", "-", "*", "/", "(", ")"};

    for (String character : chars) {
        s = s.replace(character, " " + character + " ");
    }
    s = s.trim();
    return s.split(" +");
}

Test

Compile with javac Solution.java and run with java Solution.

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