17. Letter Combinations of a Phone Number LeetCode题解

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.

Subscribe to see which companies asked this question.

题意：

给定一个数字字符串，返回其数字串代表的所有可能字母组合；

数字和字符的映射关系如图所示；

题解：

2代表abd，3代表def，23则代表abc和def的所有组合，即如样例所示；

思路上可以使用递归或者循环，这里推荐写循环，因为面试官往往不喜欢递归；

Code【Java】

public class Solution {
public List<String> letterCombinations(String digits) {
if (digits.length() < 1) {
return new LinkedList<String>();
}
String mapping[] = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
LinkedList<String> ans = new LinkedList<String>(Arrays.asList(""));
for (int i = 0; i < digits.length(); ++i) {
int num = digits.charAt(i) - '0';
while (ans.peek().length() == i) {
String cur = ans.poll();
for (int j = 0; j < mapping[num].length(); ++j) {
ans.add(cur + mapping[num].charAt(j));
}
}
}
return ans;
}
}

Code【C++】

class Solution {
public:
vector<string> letterCombinations(string digits) {
if (digits.size() < 1) {
return vector<string>();
}
string mapping[] = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
queue<string> ans;
ans.push("");
for (int i = 0; i < digits.size(); ++i) {
int num = digits[i] - '0';
while (ans.front().size() == i) {
string cur = ans.front();
for (int j = 0; j < mapping[num].size(); ++j) {
ans.push(cur + mapping[num][j]);
}
ans.pop();
}
}
vector<string> ret;
while (!ans.empty()) {
ret.push_back(ans.front());
ans.pop();
}
return ret;
}
};