227. Basic Calculator II

计算器的实现考虑使用栈。

class Solution {
    public int calculate(String s) {
        int length = s.length();
        if(s==null || length==0){
            return 0;
        }
        int num = 0;
        char sign = '+';
        Stack<Integer> stack = new Stack<Integer>();
        for(int i=0;i<length;i++){
            if(Character.isDigit(s.charAt(i))){
                num = num*10+s.charAt(i)-'0';
            }
            if(!Character.isDigit(s.charAt(i)) && ' '!=s.charAt(i) ||i==length-1 ){
                if(sign=='+'){
                    stack.push(num);
                }
                if(sign=='-'){
                    stack.push(-num);
                }
                if(sign=='*'){
                    stack.push(stack.pop()*num);
                }
                if(sign=='/'){
                    stack.push(stack.pop()/num);
                }
                sign = s.charAt(i);
                num = 0;
            }
        }
        int re = 0;
        for(int i:stack){
            re += i;
        }
        return re;
    }
}