题目：

题解：

class Solution {
public:int calculate(string s) {vector<int> stk;char preSign = '+';int num = 0;int n = s.length();for (int i = 0; i < n; ++i) {if (isdigit(s[i])) {num = num * 10 + int(s[i] - '0');}if (!isdigit(s[i]) && s[i] != ' ' || i == n - 1) {switch (preSign) {case '+':stk.push_back(num);break;case '-':stk.push_back(-num);break;case '*':stk.back() *= num;break;default:stk.back() /= num;}preSign = s[i];num = 0;}}return accumulate(stk.begin(), stk.end(), 0);}
};