题目：

题解：

class Solution {public int calculate(String s) {Deque<Integer> stack = new ArrayDeque<Integer>();char preSign = '+';int num = 0;int n = s.length();for (int i = 0; i < n; ++i) {if (Character.isDigit(s.charAt(i))) {num = num * 10 + s.charAt(i) - '0';}if (!Character.isDigit(s.charAt(i)) && s.charAt(i) != ' ' || i == n - 1) {switch (preSign) {case '+':stack.push(num);break;case '-':stack.push(-num);break;case '*':stack.push(stack.pop() * num);break;default:stack.push(stack.pop() / num);}preSign = s.charAt(i);num = 0;}}int ans = 0;while (!stack.isEmpty()) {ans += stack.pop();}return ans;}
}