79. Convert String to Integer

You are given a null-terminated string that may contain:

• An optional sign (+ or -) at the beginning
• A numeric part made of digits
• Other non-digit characters (e.g., letters, symbols) that should be ignored once they appear

Your task is to:

• Convert the initial numeric part of the string to an integer
• Stop parsing when a non-digit character is encountered
• Return the result with the correct sign

Do not use atoi(), strtol(), or similar built-in functions.

Example-1

Input: "123abc"
Output: 123

Example-2

Input: "-45!"
Output: -45

Example-3

Input: "42@#"
Output: 42

Example-4

Input: "abc123"
Output: 0

Example-5

Input: "+78x90"
Output: 78