17. Letter Combinations of a Phone Number
Medium
Given a string containing digits from 2-9 inclusive, return all possible letter combinations that the number could represent. Return the answer in any order.
A mapping of digit to letters (just like on the telephone buttons) is given below. Note that 1 does not map to any letters.
Example 1:
- Input: digits = "23"
- Output: ["ad","ae","af","bd","be","bf","cd","ce","cf"]
Example 2:
- Input: digits = ""
- Output: []
Example 3:
- Input: digits = "2"
- Output: ["a","b","c"]
Constraints:
0 <= digits.length <= 4-
digits[i]is a digit in the range['2', '9'].
Solution:
class Solution {
/**
* @param String $digits
* @return String[]
*/
function letterCombinations($digits) {
if (empty($digits)) {
return [];
}
$letters = ["", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"];
$ans = [];
$curr = '';
$this->findCombinations(0, $digits, $letters, $curr, $ans);
return $ans;
}
private function findCombinations($start, $nums, $letters, &$curr, &$ans) {
if (strlen($curr) == strlen($nums)) {
$ans[] = $curr;
return;
}
for ($i = $start; $i < strlen($nums); $i++) {
$n = intval($nums[$i]);
for ($j = 0; $j < strlen($letters[$n]); $j++) {
$ch = $letters[$n][$j];
$curr .= $ch;
$this->findCombinations($i + 1, $nums, $letters, $curr, $ans);
$curr = substr($curr, 0, -1);
}
}
}
}
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