题目描述：
Implement a basic calculator to evaluate a simple expression string.
The expression string may contain open ( and closing parentheses ), the plus + or minus sign -, non-negative integers and empty spaces .
You may assume that the given expression is always valid.
Some examples:
1 + 1 = 2
2-1 + 2 = 3
(1+(4+5+2)-3)+(6+8) = 23
思路：
1. 两个栈，1个numStack存数字，1个opStack存(,),+,-
2. 解析数字过程中，注意数字可能是多位数
3. 如果是+,(,或-，直接入栈
4. 如果是)，判断当前opStack最上操作符是否为'('，如果是直接弹出（标记为弹出），如果opStack.Peek()不为'('且opStack中有元素，则进行循环计算：
4.1 opStack弹出1个，numStack弹出2个，计算结果入numStack
4.2 最后判断，如果最上层为'('且仍未弹出则弹出这个'('
5.最后需要对余下opStack和numStack中的元素进行计算，结果入numStack(与4.1过程一样)
最后弹出numStack的结果即可，
LeetCode Basic Caculator
。实现代码：
public class Solution {public int Calculate(string s) { // two stack var pStack = new Stack<string>(); var numStack = new Stack<int>(); var n = ; // if number , push number stack for(var i = 0;i < s.Length; i++){ if(s[i] == ' '){ continue; } // if (, +,- , push op stack if(s[i] == '(' || s[i] == '+' || s[i] == '-'){ opStack.Push(s[i].ToString()); } // if ). pop op stack, pop 2 from number stack, until reach '(' else if(s[i] == ')'){ var poped = false; if(opStack.Count > 0 && opStack.Peek() == (){ poped = true; opStack.Pop(); } while(opStack.Count > 0 && opStack.Peek() != (){ var n1 = numStack.Pop(); var n2 = numStack.Pop(); var o = opStack.Pop(); numStack.Push(Calc(n1, n2, o)); } if(!poped && opStack.Count > 0 && opStack.Peek() == (){ opStack.Pop(); } } // push num into numStack, try calc until reach '(' else { // parse out number var valid = 0123456789; if(valid.Contains(s[i])){ n += s[i]; if(i == s.Length - 1 || !valid.Contains(s[i+1])){ numStack.Push(int.Parse(n)); n = ; while(opStack.Count > 0 && opStack.Peek() != () { var o = opStack.Pop(); var n1 = numStack.Pop(); var n2 = numStack.Pop(); numStack.Push(Calc(n1,n2,o)); } } } } } // pop the rest ops in opstack and numbers in number stack while(opStack.Count > 0){ var o = opStack.Pop(); if(o != + && o != -){ continue; } var n1 = numStack.Pop(); var n2 = numStack.Pop(); numStack.Push(Calc(n1,n2,o)); } return numStack.Pop();} public int Calc(int n1, int n2, string op){ switch(op) { case + : return n1 + n2; case - : return n2 - n1; default : throw new NotSupportedException(); }}}</int></string>