LeetCode -- Basic Caculator

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。

实现代码：

public class Solution {
public int Calculate(string s)
{
// two stack
var opStack = 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();
}
}

}