Evaluate Reverse Polish Notation

Evaluate postfix notation from left to right with a stack of integers. When a token is a number, parse and push it. When it is an operator, first ensure at least two operands exist, then pop the right operand followed by the left operand: subtraction and division are left op right, so reversing those pops is a common bug. Compute the result and push it back. After all tokens, a well-formed expression has exactly one value. Explain the invariant: after processing any prefix, the stack holds the values of subexpressions that have not yet been consumed. Check negative literals, a single number, division that truncates toward zero as required by the language or specification, and malformed input if validation is in scope. Each token is pushed and popped at most once, so time is O(n) and stack space is O(n) in the worst case.