题解 | #表达式求值#

using System;
using System.Collections.Generic;

class Solution {
    /**
     * 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
     */
    
    public int solve (string s) {
        s = s.Replace(" ", "");
        Stack<int> nums = new Stack<int>();
        List<char> num = new List<char>();
        Stack<char> ops = new Stack<char>();
        char op;
        nums.Push(0);
        for(int i = 0; i < s.Length; i++){
            if(s[i] >= '0' && s[i] <= '9') num.Add(s[i]);
            else{
                if(num.Count > 0){
                    nums.Push(ToNumber(num));
                    num.Clear();
                }
                if(s[i] == '(') ops.Push('(');
                else if(s[i] == ')'){
                    while(ops.Peek() != '('){
                        DoSolve(nums, ops);
                    }
                    ops.Pop();
                }
                else if(s[i] == '+' || s[i] == '-'){
                    while(ops.Count > 0 && ((op = ops.Peek()) == '+' || op == '-' || op == '*')){
                        DoSolve(nums, ops);
                    }
                    ops.Push(s[i]);
                }
                else if(s[i] == '*'){
                    while(ops.Count > 0 && ops.Peek() == '*'){
                        DoSolve(nums, ops);
                    }
                    ops.Push(s[i]);
                }
            }
        }
        if(num.Count > 0){
            nums.Push(ToNumber(num));
            num.Clear();
        }
        while(ops.Count > 0){
            DoSolve(nums, ops);
        }
        return nums.Pop();
    }

    public int ToNumber(List<char> num){
        char[] array = num.ToArray();
        return int.Parse(new string(array));
    }

    public void DoSolve(Stack<int> nums, Stack<char> ops){
        int num2 = nums.Pop();
        int num1 = nums.Pop();
        char op1 = ops.Pop();
        switch(op1){
            case '+':
            nums.Push(num1 + num2);
            break;
            case '-':
            nums.Push(num1 - num2);
            break;
            case '*':
            nums.Push(num1 * num2);
            break;
            default:
            break;
        }
    }
}