Given a number in form of a string s. The task is to find whether the number is valid indian mobile number or not.
Rules for valid :-indian mobile number
The number should contain 10 or 11 or 12 digits.
If it contains 10 digits, then first digit should be 7 or 8 or 9.
If it contains 11 digits, then first digit should be 0 and second rule follwed.
If it contains 12 digits, then first two digits should be 91 and second rule followed
Example 1:
Input: s = "912345678999"
Output: 0
Example 2:
Input: s = "123456789"
Output: 0

Your Task:
You don't need to read or print anything. Your task is to complete the function is_valid() which takes s as input parameter and returns 1 if it is valid indian mobile number otherwise returns 0.

Expected Time Complexity: O(1)
Expected Space Complexity: O(1)
Constraints:
1 <= |s| <= 13

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