Title Description

according to Reverse Polish notation , Find the value of the expression .
Valid operators include +、-、*、/ . Each operand can be an integer , It can also be another inverse Polish expression .
Be careful The division between two integers retains only the integer part .
It can be guaranteed that the given inverse Polish expression is always valid . let me put it another way , The expression always yields a valid number and does not have a divisor of 0 The situation of .

source ： Power button （LeetCode）
link ：https://leetcode-cn.com/problems/evaluate-reverse-polish-notation
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Example 1：

 Input ：tokens = ["2","1","+","3","*"]
 Output ：9
 explain ： This formula is transformed into a common infix arithmetic expression as ：((2 + 1) * 3) = 9

Example 2：

 Input ：tokens = ["10","6","9","3","+","-11","*","/","*","17","+","5","+"]
 Output ：22
 explain ： This formula is transformed into a common infix arithmetic expression as ：
  ((10 * (6 / ((9 + 3) * -11))) + 17) + 5
= ((10 * (6 / (12 * -11))) + 17) + 5
= ((10 * (6 / -132)) + 17) + 5
= ((10 * 0) + 17) + 5
= (0 + 17) + 5
= 17 + 5
= 22

Ideas

Using the idea of stacks , Push non operators, that is, numbers, onto the stack , Every time you encounter an operator , Then take out the two elements at the top of the stack and calculate , Then press the calculation result into the stack , Until the traversal is finished , Finally, there is only one element left in the stack , This element is the result of the calculation . It will take advantage of stoi function .

	//stoi Use of functions   take string Convert to corresponding integer 
	cout<<stoi("123");//123
	cout<<stoi("12a3");//12
	cout<<stoi("a12");//error

Code ：

    int evalRPN(vector<string>& tokens) {
    
	stack<int> tmp;
	int a,b,c;
	for(auto x:tokens)
	{
    
		if(x!="+" && x!="-"&& x!="*" && x!="/")
			tmp.push(stoi(x));//stoi function   take string Converted to an integer 
		else{
    
			if(x=="+"){
     
				b=tmp.top();
				tmp.pop();
				a=tmp.top();
				tmp.pop();
				c=a+b;
				tmp.push(c);
			}
			else if(x=="-"){
     // Note the order in which the elements are calculated 
				b=tmp.top();
				tmp.pop();
				a=tmp.top();
				tmp.pop();
				c=a-b;
				tmp.push(c);
			}
			else if(x=="*"){
     
				b=tmp.top();
				tmp.pop();
				a=tmp.top();
				tmp.pop();
				c=a*b;
				tmp.push(c);
			}
			else{
     // Divide   Note the order in which the elements are calculated 
				b=tmp.top();
				tmp.pop();
				a=tmp.top();
				tmp.pop();
				c=a/b;
				tmp.push(c);
			}
		}
	}
	int ans=tmp.top();
	tmp.pop();
	return ans;
  }