Add/Sub Calculator with String Verification Follow-up

Problem: Add/Sub String Calculator (with Verification) You are given a string s representing an arithmetic expression. The expression contains only: Digits 0-9 Plus sign + Minus sign - Spaces ' ' (optional) Your task has two steps: String verification: determine whether s is a valid expression. If valid, evaluate it and print the result; otherwise print INVALID. Valid expression definition After removing spaces, s is valid iff: It is non-empty; It alternates between integers and +/- operators, i.e.: number (op number)* A number is one or more digits (no empty numbers); No consecutive operators are allowed (e.g. 1++2, 1+-2); It cannot start or end with an operator (e.g. +1, 1-); No characters other than digits, +, -, and spaces are allowed; Note: This problem has no parentheses and no unary minus (so -1+2 is invalid). Input/Output Input: one line string s Output: If valid, output the integer result Otherwise, output INVALID Constraints 1 <= len(s) <= 1e5 The result fits in a 64-bit signed integer Examples (5 tests) Input: "1 + 2 - 3" Output: 0 Input: "10-20+5" Output: -5 Input: "1++2" Output: INVALID Input: "-1+2" Output: INVALID Input: " 007 + 3 " Output: 10 Example Input 1 + 2 - 3 Output 0