基本解法，运用栈进行数据存储，设定先前符号为+，之后遍历s入栈操作

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class Solution {
    public int calculate(String s) {
        Deque<Integer> stack = new ArrayDeque<Integer>();
        char preSign = '+';
        int num = 0;
        for(int i=0;i<s.length();i++){
            if(Character.isDigit(s.charAt(i))){
                 num = num * 10 + s.charAt(i) - '0'; //是否是多位
            }
            if(!Character.isDigit(s.charAt(i)) && s.charAt(i) != ' ' || 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 = s.charAt(i);
                num = 0;
                
            }
        }
        while(!stack.isEmpty()){
            num += stack.pop();
        }
         return num;
    }
}
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