LetterCombinationsOfAPhoneNumber.java

package io.ziheng.recursion.backtracking.leetcode;

import java.util.List;
import java.util.LinkedList;
import java.util.Map;
import java.util.HashMap;

/**
 * LeetCode 17. Letter Combinations of a Phone Number
 * https://leetcode.com/problems/letter-combinations-of-a-phone-number/
 */
public class LetterCombinationsOfAPhoneNumber {
    public static void main(String[] args) {
        List<String> result = new LetterCombinationsOfAPhoneNumber()
            .letterCombinations("234");
        System.out.println(result);
        result = new LetterCombinationsOfAPhoneNumber()
            .letterCombinationsIteratively("234");
        System.out.println(result);
    }
    private List<String> resultList = new LinkedList<>();
    /**
     * 号码与字母映射表。
     */
    private static final Map<Character, String> lettersMap = new HashMap<>();
    static {
        lettersMap.put('0', " ");    // 0
        lettersMap.put('1', "");     // 1
        lettersMap.put('2', "abc");  // 2
        lettersMap.put('3', "def");  // 3
        lettersMap.put('4', "ghi");  // 4
        lettersMap.put('5', "jkl");  // 5
        lettersMap.put('6', "mno");  // 6
        lettersMap.put('7', "pqrs"); // 7
        lettersMap.put('8', "tuv");  // 8
        lettersMap.put('9', "wxyz"); // 9
    }
    public List<String> letterCombinations(String digits) {
        if (digits == null || digits.length() == 0) {
            return resultList;
        }
        findCombination(digits, 0, "");
        return resultList;
    }
    public List<String> letterCombinationsIteratively(String digits) {
        if (digits == null || digits.length() == 0) {
            return new LinkedList<>();
        }
        List<String> resultList = new LinkedList<>();
        resultList.add("");
        for (int i = 0; i < digits.length(); i++) {
            resultList = combine(lettersMap.get(digits.charAt(i)), resultList);
        }
        return resultList;
    }
    private List<String> combine(String letter, List<String> lastResult) {
        List<String> resultList = new LinkedList<>();
        for (int i = 0; i < letter.length(); i++) {
            for (String s : lastResult) {
                resultList.add(s + letter.charAt(i));
            }
        }
        return resultList;
    }
    /**
     * 递归回溯法（暴力解法）。
     *
     * 时间复杂度：O(2^n)
     * 空间复杂度：O(n)
     *
     * @param digis 原始字符串
     * @param index 字符索引
     * @param s {@code digits[0... index - 1]}的解
     * @return void
     */
    private void findCombination(String digits, int index, String s) {
        // 递归退出：无字符
        if (index == digits.length()) {
            resultList.add(s);
            return;
        }
        char c = digits.charAt(index);
        assert c >= '0' && c <= '9' && c != '1';
        String letters = lettersMap.get(c);
        for (int i = 0; i < letters.length(); i++) {
            findCombination(
                digits,
                index + 1,
                s + letters.charAt(i)
            );
        }
    }
}
/* EOF */