Encode Letters with a Telephone Keypad

Encode Letters with a Telephone Keypad

On a standard 12-key telephone keypad, letters are mapped to numbers from 2 to 9 as follows:

• 2: A, B, C
• 3: D, E, F
• 4: G, H, I
• 5: J, K, L
• 6: M, N, O
• 7: P, Q, R, S
• 8: T, U, V
• 9: W, X, Y, Z

To type a letter in "multi-tap" mode, a user presses a key repeatedly. For example, pressing 2 once produces 'A', twice produces 'B', and three times produces 'C'.

Write a function that encodes a given string into its sequence of keypad button presses according to these rules:

1. Case-Insensitive: Uppercase and lowercase letters are encoded identically (e.g., 'a' and 'A' both map to "2").
2. Ignore Spaces: Spaces in the input string should be completely ignored.
3. Separator between Consecutive Same-Key Letters: If two consecutive encoded letters originate from the same number key, you must insert a space (' ') between their digit representations to distinguish between multiple presses of the same button versus separate letters. If consecutive letters use different keys, no separator is needed.

Function Signature

def encode_keypad(text: str) -> str:
    pass

Input

• text: A string consisting of English letters and spaces.

Output

• Returns a string representing the encoded button presses.

Constraints

• 0 <= len(text) <= 10^5
• text contains only ASCII English letters ('a'-'z', 'A'-'Z') and space characters (' ').

Examples

Example 1:

Input: text = "hello"
Output: "4433555 555666"
Explanation:
- 'h' -> "44" (key 4)
- 'e' -> "33" (key 3)
- 'l' -> "555" (key 5)
- 'l' -> "555" (key 5, same key as previous letter 'l', so insert a space)
- 'o' -> "666" (key 6)
Result: "4433555 555666"

Example 2:

Input: text = "A B"
Output: "2 22"
Explanation:
- Spaces in input are ignored.
- 'A' -> "2" (key 2)
- 'B' -> "22" (key 2, same key as 'A', so insert a space)
Result: "2 22"

Example 3:

Input: text = ""
Output: ""