Given a string `expression`, write a function that validates and evaluates it. The expression represents arithmetic using only non-negative integers and the operators `+` and `-`. A valid expression must satisfy all of the following: - It contains only digits, `+`, and `-`. - It does not contain spaces, parentheses, or any other characters. - It consists of numbers separated by binary operators only, so it cannot start or end with an operator. - Two operators cannot appear next to each other. - A number cannot have leading zeros unless the number is exactly `0`. - Every parsed number must fit in the 32-bit signed integer range. - The final evaluated result must also fit in the 32-bit signed integer range. If the expression is invalid, report it as invalid. Otherwise, return the computed result. Examples of invalid inputs include: - `01+2` because `01` has a leading zero - `1++2` because two operators are adjacent - `+1-2` because the expression starts with an operator - `12a-3` because it contains an invalid character Examples of valid inputs include: - `0` - `12-3+4` - `7+0-5` Implement the parser/evaluator and make sure edge cases such as the standalone value `0` are handled correctly.