Given a digit string, return all possible letter combinations that the number could represent.
A mapping of digit to letters (just like on the telephone buttons) is given below.
Input:Digit string "23" Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].
Note:
Although the above answer is in lexicographical order, your answer could be in any order you want.
public class Solution { public List<String> letterCombinations(String digits) { List<char[]> list = new ArrayList<char[]>(); list.add(new char[]{' '}); list.add(new char[]{}); list.add(new char[]{'a','b','c'}); list.add(new char[]{'d','e','f'}); list.add(new char[]{'g','h','i'}); list.add(new char[]{'j','k','l'}); list.add(new char[]{'m','n','o'}); list.add(new char[]{'p','q','r','s'}); list.add(new char[]{'t','u','v'}); list.add(new char[]{'w','x','y','z'}); List<String> ret = new ArrayList<String>(); ret.add(""); return build(ret, list, digits, 0); } private List<String> build(List<String> ret, List<char[]> list, String digits, int i) { if (0 == digits.length()) { return new ArrayList<String>(); } if (i == digits.length()) { return ret; } int pos = digits.charAt(i)-'0'; char[] str = list.get(pos); if (str.length == 0) { return ret; } List<String> newRet = new ArrayList<String>(); for (String s : ret) { for (char ch : str) { newRet.add(s+ch); } } return build(newRet, list, digits, i+1); } }
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