Leetcode 150. Evaluate the inverse Polish expression

subject

Ideas

• Scan suffix expression , When you encounter a number, put it on the stack
• When the operator is scanned , Take out the two elements at the top of the stack for operation , Then store the results on the stack

Code

class Solution {
    
public:
    int evalRPN(vector<string>& tokens) {
    
        stack<int> st;

        for(int i = 0; i < tokens.size(); i++)
        {
    
            if(tokens[i] == "+" || tokens[i] == "-" || tokens[i] == "*" || tokens[i] == "/")
            {
    
                //  Operator encountered   Take out two top stack elements for operation   Put the result back on the stack 
                int num1 = st.top();//  Take out the top element of the stack 
                st.pop();//  Out of the stack 
                int num2 = st.top();
                st.pop();
                if(tokens[i] == "+") st.push(num1 + num2);
                if(tokens[i] == "-") st.push(num2 - num1);
                if(tokens[i] == "*") st.push(num1 * num2);
                if(tokens[i] == "/") st.push(num2 / num1);
            }
            else
            {
    
                st.push(stoi(tokens[i]));
            }
        }
        int result = st.top();
        st.pop();//  Pop up the last element in the stack  
        return result;
    }
};