Evaluate Reverse Polish Notation

Evaluate the value of an arithmetic expression in Reverse Polish Notation.

Valid operators are +, -, *, /. Each operand may be an integer or another expression.

Some examples:
  ["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9
  ["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) -> 6

Analysis

• Wiki gives pseudo code and example. Very clear. See reference

Code

public class Solution {
    public int evalRPN(String[] tokens) {
        Stack<Integer> st = new Stack<Integer>();
        for(String s:tokens){
            if(s.equals("+")){
                int j = st.pop();
                int i = st.pop();
                int k = i+j;
                st.push(k);
            }
            else if(s.equals("-")){
                int j = st.pop();
                int i = st.pop();
                int k = i-j;
                st.push(k);
            }
            else if(s.equals("*")){
                int j = st.pop();
                int i = st.pop();
                int k = i*j;
                st.push(k);
            }
            else if(s.equals("/")){
                int j = st.pop();
                int i = st.pop();
                int k = i/j;
                st.push(k);
            }
            else{
                st.push(Integer.parseInt(s));
            }
        }
        return st.pop();
    }
}

Note

• use s.equals() to compare string, do not use "=="
• stack.push(), stack.pop(), stack.peek(), stack.empty()
• int = Integer.parseInt(string)

Reference

Wiki