Letter Combinations of a Phone Number:

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.

简单讲讲这道题的思路吧。一开始想到的方法是,由给定的digits字符串的每个字符,确定对应的若干个(3或4)字符,然后通过递归,从底层开始,每次将这一层的字符和vector中的字符串组合一下,再返回给上一层。

思路是挺清晰也挺简单的,不过c++实现过程中也遇到了一点困难。

  • char转换成string的问题。递归最底层要实现这个,“”+ ch并不能做到转string,最后是用一个空的string,push_back字符得到想要的结果。
  • digits的字符对应若干个字符的问题。由于太久没有写这些东西,脑子不是很清晰,一开始也写错了,最终跟这个相关的代码也是有些乱的。

第二种是非递归的方法,每次把数字对应的3或4个字符添加到结果集合中。

最后有给出python实现的代码,非递归,非常简单。

class Solution {
public:
vector<string> letterCombinations(string digits) {
return combine(digits,0);
}
std::vector<string> combine(string digits,int len){ vector<string> re,temp;
//threeOrFour:这个字符对应着几个字符;
//ext:为了7(对应4个字符)以后的数字对应字符都+了1而定义的int,值是0或1
int threeOrFour,ext;
if(digits[len] == '7' || digits[len] == '9'){
threeOrFour = 4;
}else{
threeOrFour = 3;
}
if(digits[len] > '7'){
ext = 1;
}else{
ext = 0;
}
//ch是该数字对应的第一个字符
char ch = (digits[len] - 48) * 3 + 91 + ext;
string empty = ""; if ( len < digits.size()){
temp = combine(digits,len+1);
} //递归的最底层
if (len == digits.size() - 1){
string t;
for(int i = 0; i < threeOrFour; i++){
t = empty;
t.push_back(ch+i);
re.push_back(t);
}
return re;
}
else{
for (int i = 0; i < temp.size(); ++i){
for(int j = 0; j < threeOrFour; j++){
string str = empty;
str.push_back(ch + j);
re.push_back((str+temp[i]));
}
}
return re;
}
} };

下面是修改之后,更加简洁的版本:

class Solution {
private:
string letters[10] = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
public:
vector<string> letterCombinations(string digits) {
return Mycombine(digits,0);
}
vector<string> Mycombine(string digits,int len){
std::vector<string> re, temp;
temp.push_back("");
if(digits.empty()) return re;
if(len < digits.size() - 1){
temp = Mycombine(digits,len+1);
} string get = letters[toInt(digits[len])];
for (int i = 0; i < temp.size(); ++i){
for (int j = 0; j < get.size(); ++j){
string put ="";
put.push_back(get[j]);
put += temp[i];
re.push_back(put);
}
}
return re; }
int toInt(char ch){
return ch - 48;
}
};

迭代方法:

class Solution {
private:
string letters[10] = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
public:
vector<string> letterCombinations(string digits) {
std::vector<string> re,temp;
if(digits.empty()) return re;
re.push_back("");
//temp.push_back("");
for(int i = 0; i < digits.size(); i++){
string get = letters[toInt(digits[i])];
for(int j = 0; j < re.size(); j++){
string t = re[j];
for(int k = 0; k < get.size(); k++){
string str = t;
str.push_back(get[k]);
temp.push_back(str);
}
}
re = temp;
temp.clear();
}
return re;
} int toInt(char ch){
return ch - 48;
}
};

事实上,用python的话非常好实现,而且逻辑很清晰:

class Solution:
def letterCombinations(self, digits):
"""
:type digits: str
:rtype: List[str]
"""
if len(digits) == 0:
return []
re = ['']
chars = ['','','abc','def','ghi','jkl','mno','pqrs','tuv','wxyz']
m = {i:[ch for ch in chars[i]] for i in range(0,10)} data = [int(digits[i]) for i in range(len(digits)) ] for i in data:
temp = []
for s in re:
for j in m[i]:
temp.append(s + j)
print(j)
re = temp return re

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