Write a method that determines whether a given string is a valid IPv4 address.
A valid IPv4 address must meet all of the following criteria:
- Contains exactly 4 parts separated by dots
- Each part must be a whole number between 0 and 255 (inclusive)
- No leading zeros are allowed (the string "01" is invalid, but "0" alone is valid)
- Null or empty input is considered invalid
Examples
Input: ip = "192.168.1.1"
Output: true
Explanation: All 4 parts are valid numbers between 0 and 255
Input: ip = "256.0.0.1"
Output: false
Explanation: 256 exceeds the maximum value of 255
Input: ip = "01.1.1.1"
Output: false
Explanation: Leading zeros are not permitted in IP address octets
Input: ip = "1.2.3"
Output: false
Explanation: Only 3 parts found, a valid IPv4 address requires exactly 4
Input: ip = null
Output: false
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