17. 电话号码的字母组合
难度中等1276收藏分享切换为英文接收动态反馈
给定一个仅包含数字 2-9 的字符串，返回所有它能表示的字母组合。答案可以按 任意顺序 返回。
给出数字到字母的映射如下（与电话按键相同）。注意 1 不对应任何字母。
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示例 1：
输入：digits = "23"
输出：["ad","ae","af","bd","be","bf","cd","ce","cf"]
示例 2：
输入：digits = ""
输出：[]
示例 3：
输入：digits = "2"
输出：["a","b","c"]
提示：
0 <= digits.length <= 4-
digits[i]是范围['2', '9']的一个数字。
- 1Letter Combinations of a Phone Number
Medium
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Given a string containing digits from 2-9 inclusive, return all possible letter combinations that the number could represent. Return the answer in any order.
A mapping of digit to letters (just like on the telephone buttons) is given below. Note that 1 does not map to any letters.
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Example 1:
Input: digits = "23"
Output: ["ad","ae","af","bd","be","bf","cd","ce","cf"]
Example 2:
Input: digits = ""
Output: []
Example 3:
Input: digits = "2"
Output: ["a","b","c"]
Constraints:
0 <= digits.length <= 4-
digits[i]is a digit in the range['2', '9'].
方法：暴力递归，深度优先遍历
image-20210426084946288
class Solution {
public static char[][] phone = {
{ 'a', 'b', 'c' }, // 2 0
{ 'd', 'e', 'f' }, // 3 1
{ 'g', 'h', 'i' }, // 4 2
{ 'j', 'k', 'l' }, // 5 3
{ 'm', 'n', 'o' }, // 6
{ 'p', 'q', 'r', 's' }, // 7
{ 't', 'u', 'v' }, // 8
{ 'w', 'x', 'y', 'z' }, // 9
};
// "23"
public static List<String> letterCombinations(String digits) {
List<String> ans = new ArrayList<>();
if (digits == null || digits.length() == 0) {
return ans;
}
char[] str = digits.toCharArray();
char[] path = new char[str.length];
process(str, 0, path, ans);
return ans;
}
// str = ['2','3'] 3 3
// str[....index-1]，按出的结果是什么都在path里
// str[index...] 按完之后，有哪些组合，放入到ans里
public static void process(char[] str, int index, char[] path, List<String> ans) {
if (index == str.length) {
ans.add(String.valueOf(path));
} else {
char[] cands = phone[str[index] - '2'];
for (char cur : cands) {
path[index] = cur;
process(str, index + 1, path, ans);
}
}
}
}
image-20210426085146937
