# C++ 9ms solution, Stack

• ``````class Solution {
public:
int operation(char op, int op1, int op2){
if (op == '+') {return op1 + op2;}
if (op == '-') {return op1 - op2;}
if (op == '*') {return op1 * op2;}
if (op == '/') {return op1 / op2;}
}

int evalRPN(vector<string>& tokens) {
stack<int> operands;
for (int i = 0; i < tokens.size(); i++) {
if (tokens[i].size() == 1 && (tokens[i][0] == '+' || tokens[i][0] == '-' || tokens[i][0] == '*' || tokens[i][0] == '/')) {
int op2 = operands.top();
operands.pop();
int op1 = operands.top();
operands.pop();
operands.push(operation(tokens[i][0], op1, op2));
} else {
operands.push(stoi(tokens[i]));
}
}
return operands.top();
}
};
``````

Example:

Consider sample input: ["5", "12", "4", "3", "*", "/", "+"]

• Build a stack with the raw operands
• Working downward from the top of your stack, evaluate 2 operands for each consecutive expression, and add the result as a new operand.
• When the loop concludes, there will be one operand remaining on the stack (the solution)

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