0227. Basic Calculator II

https://leetcode.com/problems/basic-calculator-ii

Description

Given a string s which represents an expression, evaluate this expression and return its value.

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 = "3+2*2"
**Output:** 7

Example 2:

**Input:** s = " 3/2 "
**Output:** 1

Example 3:

**Input:** s = " 3+5 / 2 "
**Output:** 5

Constraints:

• 1 <= s.length <= 3 * 105

• s consists of integers and operators ('+', '-', '*', '/') separated by some number of spaces.

• s represents a valid expression.

• All the integers in the expression are non-negative integers in the range [0, 231 - 1].

• The answer is guaranteed to fit in a 32-bit integer.

ac1

intuitive solution, so disgusting.

class Solution {
    public int calculate(String s) {
        Queue<String> queue = new LinkedList<String>();
        Deque<String> deque = new ArrayDeque<String>();

        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);
            if (c == ' ') continue;
            if (c != '+' && c != '-' && c != '*' && c != '/') {
                sb.append(c);
            } else {
                queue.offer(sb.toString());
                queue.offer(String.valueOf(c));
                sb.setLength(0);
            }
        }
        queue.offer(sb.toString());

        while (!queue.isEmpty()) {
            String c = queue.poll();
            if (c.equals("*") || c.equals("/")) {
                String operand = queue.poll();
                String num = deque.pollLast();
                switch (c) {
                    case "*":
                        int tmp = Integer.parseInt(num) * Integer.parseInt(operand);
                        deque.offer("" + tmp);
                        break;
                    case "/":
                        tmp = Integer.parseInt(num) / Integer.parseInt(operand);
                        deque.offer("" + tmp);
                }
            } else {
                deque.offer(c);
            }
        }

        int val = Integer.parseInt(deque.poll());
        while (!deque.isEmpty()) {
            String operator = deque.poll();
            String operand = deque.poll();
            switch(operator) {
                case "+":
                    val += Integer.parseInt(operand);
                    break;
                case "-":
                    val -= Integer.parseInt(operand);
            }
        }

        return val;
    }
}

ac2:

class Solution {
    public int calculate(String s) {
        Stack<Integer> nums = new Stack<>();
        nums.push(1);
        Character oper = '*';
        int n = 0;

        s.replace(" ", "");
        for (int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);
            // if (c == ' ') continue;
            if (Character.isDigit(c)) {
                n = n * 10 + c - '0';
            }
            // can't else if, because need to handle last number of expression
            if (i == s.length() - 1 || c == '+' || c == '-' || c == '*' || c == '/') {
                switch (oper) {
                    case '+': nums.push(n);break;
                    case '-': nums.push(-n);break;
                    case '*': nums.push(nums.pop() * n);break;
                    case '/': nums.push(nums.pop() / n);break;
                }
                n = 0;
                oper = c;
            }
        }

        // add values in stack
        int res = 0;
        for (int i : nums) res += i;

        return res;
    }
}
/*
The essential idea is the same as 2 stack #772, keep track of numbers and operators. This one is simplified, one variable to record operator is fire enough.
*/