题解 | #表达式求值#

using System;
using System.Collections.Generic;
using System.Linq;


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) {
        // write code here
        return cal(s);
    }
    
     public int cal(string s)
        {
            int sum = 0;
            char sign = '+';
            int i = 0;
            Stack<int> nums = new Stack<int>();
            while(i<s.Length)
            {             
                if(s[i]=='(')
                {
                    int flag = 1;
                    int startIndex = i + 1;
                    while(flag>0)
                    {
                        i++;
                        if (s[i] == '(')
                            flag++;
                        if (s[i] == ')')
                            flag--;
                    }
                    sum=cal(s.Substring(startIndex,i-startIndex));
                    i--;  //这儿做减号是为了下一轮循环到')'时.将运算结果入栈
                }
                else if(char.IsDigit(s[i]))
                {
                    sum = sum * 10 + (s[i] - '0');
                }
                if(!char.IsDigit(s[i])  || i==s.Length-1)
                {
                    if (sign == '+')
                        nums.Push(sum);
                    if (sign == '*')
                        nums.Push(sum * nums.Pop());
                    if (sign == '-')
                        nums.Push(sum*(-1));
                    sum = 0;
                    sign = s[i];
                }
                i++;
            }
            return nums.Aggregate(0, (total, next) => total + next);
        }   
}