Given a string of digits representing a phone number of variable length (N), find all possible valid words that can be formed using the traditional phone number letter mapping. This mapping is as follows:

• 2: 'abc'
• 3: 'def'
• 4: 'ghi'
• 5: 'jkl'
• 6: 'mno'
• 7: 'pqrs'
• 8: 'tuv'
• 9: 'wxyz'

Functionality

You are required to build a build_options function which returns all valid words that can be spelled using the digits in the given phone number.

Assumptions

• A valid word dictionary (like /var/lib/dict from Linux systems) is available.
• A function load_dict() exists, which loads the dictionary into a data structure (e.g., a list). This function returns a list of words, for example: ['a', 'and', 'also', ..., 'zebra'].

Example

Given build_options('76278'), the function should return valid words such as ['roast', 'smart', 'snast'].

Notes

• The phone number can be of any length.
• The function should only return words that are present in the provided dictionary.