227 基本计算器 II

https://leetcode.cn/leetbook/read/top-interview-questions-hard/xwsg7t/

cost: 22m50s

class Solution {
public:
    int calculate(string s) {
        stack<char> operaters;
        stack<int> operands;
        for (int i = 0; i < s.size();) {
            char ch = s[i];
            if (ch == ' ') {
                i++;
                continue;
            }
            if (ch == '+' || ch == '-') {
                if (!operaters.empty()) {
                    char op = operaters.top();
                    operaters.pop();
                    int v2 = operands.top();
                    operands.pop();
                    int v1 = operands.top();
                    operands.pop();
                    if (op == '+') {
                        v1 += v2;
                    } else {
                        v1 -= v2;
                    }
                    operands.push(v1);
                }
                operaters.push(ch);
                i++;
                continue;
            }
            if (ch == '*' || ch == '/') {
                i++;
                int v1 = operands.top();
                operands.pop();
                int v2 = this->parseInt(s, i);
                if (ch == '*') {
                    v1 *= v2;
                } else {
                    v1 /= v2;
                }
                operands.push(v1);
                continue;
            }
            if (std::isdigit(ch)) {
                int operand = this->parseInt(s, i);
                operands.push(operand);
            }
        }
        while (!operaters.empty()) {
            char op = operaters.top();
            operaters.pop();
            int v2 = operands.top();
            operands.pop();
            int v1 = operands.top();
            operands.pop();
            if (op == '+') {
                v1 += v2;
            } else {
                v1 -= v2;
            }
            operands.push(v1);
        }
        return operands.top();
    }

private:
    int parseInt(string& s, int& i) {
        while (i < s.size() && s[i] == ' ') {
            i++;
        }
        int res = 0;
        while (i < s.size() && std::isdigit(s[i])) {
            res *= 10;
            res += s[i] - '0';
            i++;
        }
        return res;
    }
};