227. Basic Calculator II

Implement a basic calculator to evaluate a simple expression string.

The expression string contains only non-negative integers, +, -, *, / operators and empty spaces . The integer division should truncate toward zero.

You may assume that the given expression is always valid.

Some examples:

"3+2*2" = 7
" 3/2 " = 1
" 3+5 / 2 " = 5

fb： 

例如，Input是：13*(1256*31-(33/2))-5/8+9，然后output是计算出来的integer结果

using two stacks，  one for operands and one for operators.

 

 

public int calculate(String s) {
    if (s == null) return 0;
    s = s.trim().replaceAll(" +", "");
    int length = s.length();
    
    int res = 0;
    long preVal = 0; // initial preVal is 0
    char sign = '+'; // initial sign is +
    int i = 0;
    while (i < length) {
        long curVal = 0;
        while (i < length && (int)s.charAt(i) <= 57 && (int)s.charAt(i) >= 48) { // int
            curVal = curVal*10 + (s.charAt(i) - '0');
            i++;
        }
        if (sign == '+') {
            res += preVal;  // update res
            preVal = curVal;
        } else if (sign == '-') {
            res += preVal;  // update res
            preVal = -curVal;
        } else if (sign == '*') {
            preVal = preVal * curVal; // not update res, combine preVal & curVal and keep loop
        } else if (sign == '/') {
            preVal = preVal / curVal; // not update res, combine preVal & curVal and keep loop
        }
        if (i < length) { // getting new sign
            sign = s.charAt(i);
            i++;
        }
    }
    res += preVal;
    return res;
}

 public int calculate(String s) {
        if (s == null) {
            return 0;
        }    
        Queue<Character> q = new LinkedList<>();
        for (char c : s.toCharArray()) {
            q.offer(c);
        }
        q.offer('+');
        return cal(q);
    }
    
    private int cal(Queue<Character> q) {
        char sign = '+';
        int num = 0;
        Stack<Integer> stack = new Stack<>();
        while (!q.isEmpty()) {
            char c = q.poll();
            if (c == ' ') {
                continue;
            }
            if (Character.isDigit(c)) {
                num = 10 * num + c - '0';
            } else if (c == '(') {
                num = cal(q);
            } else {
                if (sign == '+') {
                    stack.push(num);
                } else if (sign == '-') {
                    stack.push(-num);
                } else if (sign == '*') {
                    stack.push(stack.pop() * num);
                } else if (sign == '/') {
                    stack.push(stack.pop() / num);
                }
                num = 0;
                sign = c;
                if (c == ')') {
                    break;
                }
            }
        }
        int sum = 0;
        while (!stack.isEmpty()) {
            sum += stack.pop();
        }
        return sum;
    }