LeetCode -- Basic Calculator II

LeetCode -- Basic Calculator II
Description:


Implement a basic calculator to evaluate a simple expression string.


The expression string contains only non-negative integers, +,-, *,/operators and empty spaces. The integer division shoshould truncate toward zero.


You may assume that the given expression is always valid.


Some examples:
"3 + 2*2" = 7
"3/2" = 1
& Quot; 3 + 5/2 & quot; = 5


It is to parse the operands and operators for one expression, and perform four arithmetic operations, but not including parentheses.


Ideas:
1. Remove white space characters
2. Multi-digit words
3. If it is '*' or '/', calculate it directly; otherwise, the data is written into the stack.
4. Since only the addition and subtraction operators are left, the results can be calculated in order.




Implementation Code:

public int Calculate(string s){var exp = s.Where(c => c != ' ').ToList();var stackOp = new Stack
 
  ();var stackNum = new Stack
  
   ();var ops = new []{'+','-','*','/'};var n = "";for(var i = 0;i < exp.Count; i++){if(ops.Contains(exp[i])) // detected operator{stackOp.Push(exp[i]);}else{// parse the numberfor(var j = i;j < exp.Count; j++){if(!ops.Contains(exp[j])){n += exp[j];}else{break;}}var n1 = int.Parse(n);// if top is '*' or '/' , immediately calculateif(stackOp.Count > 0 && (stackOp.Peek() == '*' || stackOp.Peek() == '/')){var n2 = stackNum.Pop();stackNum.Push(Calc(n2, n1, stackOp.Pop()));}else{stackNum.Push(n1);}i += n.Length - 1;n = "";}}// since now we only left '+' and '-'// we should reverse the order (both operators and numbers) back to where they werevar stackOp1 = stackOp.Reverse();var stackNum1 = new Stack
   
    (stackNum);var count = 0;foreach(var op in stackOp1){var n1 = stackNum1.Pop();var n2 = stackNum1.Pop();stackNum1.Push(Calc(n1 , n2, op));count += 2;}return stackNum1.Pop();}private int Calc(int n1 , int n2, char op){switch(op){case '+':return n1 + n2;case '-':return n1 - n2;case '*':return n1 * n2;case '/':return n1 / n2;default :throw new ArgumentException(string.Format("unexpected operator : {0}", op));}}