Backtrack choosing one letter for each digit.
Map digits to letters. DFS index i:
digits[i], append to path and recurse.Cartesian product of letter sets per digit; backtracking enumerates all combinations.
Backtrack choosing one letter for each digit.
Map digits to letters. DFS index i:
digits[i], append to path and recurse.Cartesian product of letter sets per digit; backtracking enumerates all combinations.