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| class Solution {
private static final Map<Character, List<Character>> num2Alpha = new HashMap<>();
private static final StringBuilder combination = new StringBuilder();
private void initMap() {
num2Alpha.put('2', List.of('a', 'b', 'c'));
num2Alpha.put('3', List.of('d', 'e', 'f'));
num2Alpha.put('4', List.of('g', 'h', 'i'));
num2Alpha.put('5', List.of('j', 'k', 'l'));
num2Alpha.put('6', List.of('m', 'n', 'o'));
num2Alpha.put('7', List.of('p', 'q', 'r', 's'));
num2Alpha.put('8', List.of('t', 'u', 'v'));
num2Alpha.put('9', List.of('w', 'x', 'y', 'z'));
}
public List<String> letterCombinations(String digits) {
List<String> ans = new LinkedList<>();
if (digits.length() == 0) {
return ans;
}
initMap();
backtrack(ans, digits, 0);
return ans;
}
private void backtrack(List<String> ans, String digits, int index) {
if (index == digits.length()) {
ans.add(combination.toString());
return;
}
var letters = num2Alpha.get(digits.charAt(index));
for (Character letter : letters) {
combination.append(letter);
backtrack(ans, digits, index + 1);
combination.deleteCharAt(index);
}
}
}
|
