题解 | #表达式求值#

#include<bits/stdc++.h>
#include <cctype>
#include <memory>
#include <queue>
using namespace std;
class Solution {
  public:
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 返回表达式的值
     * @param s string字符串 待计算的表达式
     * @return int整型
     */
    int solve(string s) {
        stack<int> numbers;
        stack<char> operators;

        for (int i = 0; i < s.size(); ++i) {
            if (isdigit(s[i])) {
                int num = 0;
                while (i < s.size() && isdigit(s[i])) {
                    num = num * 10 + (s[i++] - '0');
                }
                --i;
                numbers.push(num);
            } else if (s[i] == '(')
                operators.push(s[i]);
            else if (s[i] == ')') {
                while (operators.top() != '(' && !operators.empty()) {
                    int op2 = numbers.top();
                    numbers.pop();
                    int op1 = numbers.top();
                    numbers.pop();

                    int op_ret = operators.top();
                    operators.pop();

                    numbers.push(Operation(op1, op2, op_ret));
                }
                operators.pop();
            } else {
                if (!operators.empty() && Priority(operators.top()) >= Priority(s[i])) {
                    int op2 = numbers.top();
                    numbers.pop();
                    int op1 = numbers.top();
                    numbers.pop();

                    int op_ret = operators.top();
                    operators.pop();

                    numbers.push(Operation(op1, op2, op_ret));
                }
                operators.push(s[i]);
            }
        }

        while (!operators.empty()) {
            int op2 = numbers.top();
            numbers.pop();
            int op1 = numbers.top();
            numbers.pop();

            int op_ret = operators.top();
            operators.pop();

            numbers.push(Operation(op1, op2, op_ret));
        }

        return numbers.top();
    }


    int Operation(int ret1, int ret2, char ret3) {
        switch (ret3) {
            case '+':
                return ret1 + ret2;
            case '-':
                return ret1 - ret2;
            case '*':
                return ret1 * ret2;
        }
        return 0;
    }

    int Priority(char ret) {
        if (ret == '*') return 2;
        if (ret == '+' || ret == '-') return 1;
        return 0;
    }

};