Leetcode 150. Evaluate Reverse Polish Notation || Code + Explanation + Example Walkthrough

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

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

Note that division between two integers should truncate toward zero.

It is guaranteed that the given RPN expression is always valid. That means the expression would always evaluate to a result, and there will not be any division by zero operation.



Example 1:

Input: tokens = ["2","1","+","3","*"]
Output: 9
Explanation: ((2 + 1) * 3) = 9
Example 2:

Input: tokens = ["4","13","5","/","+"]
Output: 6
Explanation: (4 + (13 / 5)) = 6
Example 3:

Input: tokens = ["10","6","9","3","+","-11","*","/","*","17","+","5","+"]
Output: 22
Explanation: ((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

[abaytekle]

very nice explanation you deserve more than this 🙏🙏🙏

[venkatakalyan]

Superb Explanation👌.Neat and clean. You deserves this

[KRajesh-ejdy]

Thank you soo much in a single attempt i have done this.Thanks soo much

[shubhamsukum]

Use "long long int" instead of "int" those who are getting RUNTIME ERROR

[anujkanojia]

terminate called after throwing an instance of 'std::invalid_argument'
what(): stoi err is showing

[none]

Nice explanation but its saying wrong answer for test case 2: ["4", "13", "5", "/", "+"]. My answer is 15 but expected is 6

[Star_Bawa]

i am doing this code in java and it is giving an empty stack exception I have added the code below please help

class Solution {
public int evalRPN(String[] tokens) {
Stack<Integer> st=new Stack<Integer>();
if(tokens.length==0)
return 0;
for(String s:tokens)
{
if(s.equals("+"))
{
int a=st.peek();
st.pop();
int b=st.peek();
st.pop();
st.push(a+b);
}
else if(s.equals("-"))
{
int a=st.peek();
st.pop();
int b=st.peek();
st.pop();
st.push(a-b);
}
else if(s.equals("*"))
{
int a=st.peek();
st.pop();
int b=st.peek();
st.pop();
st.push(a*b);
}
else if(s.equals("/"))
{
int a=st.peek();
st.pop();
int b=st.peek();
st.pop();
if(a>b &&b!=0)
st.push(a/b);
else if(b>a && a!=0)st.push(b/a);
}
else
{
int a=Integer.parseInt(s);
st.push(a);
}
}
return st.pop();
}
}