public static void main(String[] args){

String s = "46";

System.out.println(calculate(s));

}

public static int calculate(String s){

int res=0;

Stack<Integer> st1 = new Stack<>();

// don't need a sign stack. keep track of the last sign

// 注意如果最后一个字符是字母，不要忘了

char sign = '+';

int cur = 0;

for(int i = 0;i < s.length(); i++){

char c = s.toCharArray()[i];

if(Character.isDigit(c)){

System.out.println("cur:" + cur);

System.out.println("c:" + c);

cur = cur*10 + Integer.parseInt(c+"");

System.out.println(cur);

}

if((!Character.isDigit(c) && c != ' ') || (i == s.length()-1)){

// c could be +,-,/,*,' '

if(sign == '+') st1.push(cur);

if(sign == '-') st1.push(-cur);

if(sign == '*') st1.push(st1.pop() * cur);

if(sign == '/') st1.push(st1.pop() / cur);

sign = c;

cur = 0;

}

System.out.println(st1);

}

while(!st1.isEmpty()){

int n = st1.pop();

res += n;

}

return res;

}