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
Not a tricky problem. Using a stack can make your life much easier.
However, one thing I should always remember: use .equals() to compare strings instead of ==
"The function checks the actual contents of the string, the
==
operator checks whether the references to the objects are equal. ". public class RPN { public int evalRPN(String[] tokens) { if (tokens == null) return 0; String operators = "+-*/"; Stackstack = new Stack (); for (String token : tokens) { if (!operators.contains(token)) { stack.push(Integer.valueOf(token)); continue; } int b = stack.pop(); int a = stack.pop(); if (token.equals("+")) stack.push(a + b); else if (token.equals("-")) stack.push(a - b); else if (token.equals("*")) stack.push(a * b); else stack.push(a / b); } return stack.pop(); } }
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