LeetCode 面试题 16.26. 计算器

将数字保存到栈中

class Solution {
    public int calculate(String s) {
        Deque<Integer> stack = new LinkedList<Integer>();
        char preSign = '+';
        int sum = 0;
        int num = 0;

        for (int i = 0; i < s.length(); i++) {        
            char c = s.charAt(i);
            if (Character.isDigit(c)) {
                num = num * 10 + (c - '0');
            }
            if (c == '+' || c == '-' || c == '*' || c == '/' || i == s.length()-1) {
                switch(preSign) {
                    case '+':
                        stack.push(num);
                        break;
                    case '-':
                        stack.push(-num);
                        break;
                    case '*':
                        stack.push(stack.pop() * num);
                        break;
                    case '/':
                        stack.push(stack.pop() / num);
                        break;
                }
                preSign = c;
                num = 0; 
            }
        }
        while (!stack.isEmpty()) {
            sum += stack.pop();
        }
        return sum;
    }    
}