Traversing through vector does not work, returns more elements when trying to print it

Sample input: tokens = ["2","1","+","3","*"]
Sample output: 9
Here is the code:

class Solution {
public:
    int evalRPN(vector<string>& tokens) {
        stack<int> s1;
        for(size_t i = 0; i < size(tokens); i++){
          if(tokens[i]!= "+"||tokens[i]!= "-"||tokens[i]!= "*"||tokens[i]!= "/") {
              cout << tokens[i]; //prints '21+'???
              s1.push(stoi(tokens[i]));
          }
          if(tokens[i]== "+"){
            int tmp1 = s1.top();
            s1.pop();
            int tmp2 = s1.top();
            s1.pop();
            int res = tmp1 + tmp2;
            s1.push(res);
          }
          if(tokens[i]== "-"){
            int tmp1 = s1.top();
            s1.pop();
            int tmp2 = s1.top();
            s1.pop();
            int res = tmp1 - tmp2;
            s1.push(res);
          }
          if(tokens[i]== "*"){
            int tmp1 = s1.top();
            s1.pop();
            int tmp2 = s1.top();
            s1.pop();
            int res = tmp1 * tmp2;
            s1.push(res);
          }
          if(tokens[i]== "/"){
            int tmp1 = s1.top();
            s1.pop();
            int tmp2 = s1.top();
            s1.pop();
            int res = tmp1 / tmp2;
            s1.push(res);
          }
        }
        int tmp = s1.top();
        return tmp;
    }
};

As you can see in the first iteration of for loop(i=0) for some reason when I try to get the first element of vector ("2") and perform operations with it it does not work because the program thinks that "21+" is the first element of vector.