Group words that map to same phone digits

Problem

On a classic phone keypad (T9), letters map to digits:

• 2: ABC , 3: DEF , 4: GHI , 5: JKL ,
• 6: MNO , 7: PQRS , 8: TUV , 9: WXYZ

Given a list of lowercase words, convert each word into its digit string (e.g., "tree" -> "8733").

Return all groups of words that map to the same digit string (i.e., collisions). Only include groups of size >= 2.

Input / Output

• Input: words: string[] (lowercase a–z)
• Output: A list of groups, where each group is a list of words sharing the same digit encoding.

Constraints (typical)

• 1 <= len(words) <= 2 * 10^5
• 1 <= len(word) <= 30

Examples

• Input: ["tree", "used", "apple", "vase"]
  • tree -> 8733 , used -> 8733 , apple -> 27753 , vase -> 8273
  • Output: [["tree", "used"]]