Letter Combinations of a Phone Number

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.

Tips:

FIFO queue, faster: 以234为例，先加入abc，然后提出a，加d，e，f存，再提出b,c,加d,e,f存，再提出ad，存adg，adh，adi,以此类推。

DFS：就是普通的dfs。。。

Code：

BFS

public class Solution {
    public List<String> letterCombinations(String digits) {
        List<String> res = new ArrayList<>();
        if (digits == null || digits.length() == 0) return res;
        String[] keyboard = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
        res.add("");
        for (int i = 0; i < digits.length(); i++) {
            int curt = digits.charAt(i) - '0';
            while (res.get(0).length() == i) {
                String temp = res.remove(0);
                for (char c : keyboard[curt].toCharArray()) {
                    res.add(temp + c);
                }
            }
        }
        return res;
    }
}

DFS：

stringBuilder版

public class Solution {
    String[] keyboard = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
    public List<String> letterCombinations(String digits) {
        List<String> res = new ArrayList<>();
        if (digits == null || digits.length() == 0) return res;
        dfs(digits, res, 0, new StringBuilder());
        return res;
    }

    private void dfs(String digits, List<String> res, int index, StringBuilder prev) {
        if (index == digits.length()) {
            res.add(prev.toString());
            return;
        }
        String letters = keyboard[digits.charAt(index) - '0'];
        for (char c : letters.toCharArray()) {
            dfs(digits, res, index + 1, prev.append(c));
            prev.deleteCharAt(prev.length() - 1);
        }
    }
}