Letter Combination Builder
Problem Description
On a classic phone keypad, each digit maps to a set of letters:
- `2` → `"abc"`, `3` → `"def"`, `4` → `"ghi"`, `5` → `"jkl"`
- `6` → `"mno"`, `7` → `"pqrs"`, `8` → `"tuv"`, `9` → `"wxyz"`
Given a string `digits` containing digits from `2–9`, return **all possible letter combinations** that the digit sequence could represent. Return the answer in any order.
If `digits` is empty, return an empty list.
**Example 1:**
```
Input: digits = "34"
Output: ["dg","dh","di","eg","eh","ei","fg","fh","fi"]
```
**Example 2:**
```
Input: digits = "7"
Output: ["p","q","r","s"]
```
Constraints
- 0 <= digits.length <= 4
- digits[i] is a digit in the range ['2', '9']
Follow-up
Can you solve this iteratively without recursion, building combinations level by level?
Hints
Try the problem first. If you get stuck, you can reveal hints one at a time.
Solution
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