17. Letter Combinations of a Phone Number

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本文介绍了一种算法，用于根据电话按键上的数字返回所有可能的字母组合。通过递归方法实现，详细展示了Java代码示例。

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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.

Solution:

Tips:

recursion

Java Code :

public class Solution {
    public List<String> letterCombinations(String digits) {
        List<String> ls = new ArrayList<>();
        char[] dc = digits.toCharArray();

        for (int i = 0; i < dc.length; i++) {
            if (!(dc[i] > '1' && dc[i] <= '9')) {
                return ls;
            }
        }

        Map<Integer, String> mcs = new HashMap<>();
        mcs.put(2, "abc");
        mcs.put(3, "def");
        mcs.put(4, "ghi");
        mcs.put(5, "jkl");
        mcs.put(6, "mno");
        mcs.put(7, "pqrs");
        mcs.put(8, "tuv");
        mcs.put(9, "wxyz");

        String str = "";
        List<String> result = new ArrayList<>();
        letterCombinations(digits, str, 0, mcs, result);

        return result;
    }
    
    public void letterCombinations(String digits, String str, int k, Map<Integer, String> mcs, List<String> result) {
        if (k == digits.length()) {
            if (str != "") {
                result.add(str);
            }
            return;
        }

        for (int i = k; i < digits.length(); i++) {
            String cs = mcs.get(digits.charAt(i) - '0');

            for (int j = 0; j < cs.length(); j++) {
                if (str.length() == i) {
                    letterCombinations(digits, str + cs.charAt(j), i + 1, mcs, result);
                }
            }
        }
    }
}