17. Letter Combinations of a Phone Number

Given a string of digits 2–9, return all letter combinations the number could represent on a phone keypad, in any order.

Pick a letter for the current digit, recurse on the next digit, then undo. When every digit has a letter, record the string.

1function letterCombinations(digits: string): string[] {
2  if (!digits) return [];
3  const keys = ["", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"];
4  const res: string[] = [], path: string[] = [];
5  function dfs(i: number) {
6    if (i === digits.length) { res.push(path.join("")); return; }
7    for (const c of keys[+digits[i]]) {
8      path.push(c);
9      dfs(i + 1);
10      path.pop();
11    }
12  }
13  dfs(0);
14  return res;
15}

path = aletter = a

digits

results

empty

Digit 2 → 'a'.

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