Letter Combinations of a Phone Number

描述

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.

分析

无

递归

// Letter Combinations of a Phone Number
// 时间复杂度O(3^n)，空间复杂度O(n)
public class Solution {
    private static final String[] keyboard =
            new String[]{ " ", "", "abc", "def", // '0','1','2',...
            "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz" };

    public List<String> letterCombinations(String digits) {
        List<String> result = new ArrayList<>();
        if (digits.isEmpty()) return result;
        dfs(digits, 0, "", result);
        return result;
    }

    private static void dfs(String digits, int cur, String path,
                            List<String> result) {
        if (cur == digits.length()) {
            result.add(path);
            return;
        }
        final String str = keyboard[digits.charAt(cur) - '0'];

        for (char c : keyboard[digits.charAt(cur) - '0'].toCharArray()) {
            dfs(digits, cur + 1, path + c, result);
        }
    }
}

迭代

// Letter Combinations of a Phone Number
// 时间复杂度O(3^n)，空间复杂度O(1)
public class Solution {
    private static final String[] keyboard =
            new String[]{ " ", "", "abc", "def", // '0','1','2',...
                    "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz" };

    public List<String> letterCombinations(String digits) {
        if (digits.isEmpty()) return new ArrayList<>();
        List<String> result = new ArrayList<>();
        result.add("");
        for (char d : digits.toCharArray()) {
            final int n = result.size();
            final int m = keyboard[d - '0'].length();

            // resize to n * m
            for (int i = 1; i < m; ++i) {
                for (int j = 0; j < n; ++j) {
                    result.add(result.get(j));
                }
            }

            for (int i = 0; i < result.size(); ++i) {
                result.set(i, result.get(i) + keyboard[d - '0'].charAt(i/n));
            }
        }
        return result;
    }
}