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

[LeetCode]Letter Combinations of a Phone Number题解的更多相关文章

  1. LeetCode: Letter Combinations of a Phone Number 解题报告

    Letter Combinations of a Phone Number Given a digit string, return all possible letter combinations ...

  2. [LeetCode] Letter Combinations of a Phone Number 电话号码的字母组合

    Given a digit string, return all possible letter combinations that the number could represent. A map ...

  3. LeetCode——Letter Combinations of a Phone Number

    Given a digit string, return all possible letter combinations that the number could represent. A map ...

  4. [LeetCode] Letter Combinations of a Phone Number

    Given a digit string, return all possible letter combinations that the number could represent. A map ...

  5. [LeetCode] Letter Combinations of a Phone Number(bfs)

    Given a digit string, return all possible letter combinations that the number could represent. A map ...

  6. LeetCode Letter Combinations of a Phone Number (DFS)

    题意 Given a digit string, return all possible letter combinations that the number could represent. A ...

  7. [LeetCode] Letter Combinations of a Phone Number 回溯

    Given a digit string, return all possible letter combinations that the number could represent. A map ...

  8. LeetCode Letter Combinations of a Phone Number 电话号码组合

    题意:给一个电话号码,要求返回所有在手机上按键的组合,组合必须由键盘上号码的下方的字母组成. 思路:尼玛,一直RE,题意都不说0和1怎么办.DP解决. class Solution { public: ...

  9. leetcode Letter Combinations of a Phone Number python

    class Solution(object): def letterCombinations(self, digits): """ :type digits: str : ...

随机推荐

  1. Vue 框架 笔记

    1.兼容 2.性能优化   vuejs ******作者:尤雨溪 *** *******MVVM框架: M:model层 数据层 数据的增删改查 V:view视图层 类似于html的模板 vm:vie ...

  2. 如何学习sql语言?

    如何学习 SQL 语言? https://www.zhihu.com/question/19552975 没有任何基础的人怎么学SQL? https://www.zhihu.com/question/ ...

  3. 关于gcd和exgcd的一点心得,保证看不懂(滑稽)

    网上看了半天……还是没把欧几里得算法和扩展欧几里得算法给弄明白…… 然后想了想自己写一篇文章好了…… 参考文献:https://www.cnblogs.com/hadilo/p/5914302.htm ...

  4. AI 的下一个重大挑战:理解语言的细微差别

    简评:人类语言非常博大精妙,同一句话在不同的语境下,就有不同的含义.连人类有时候都不能辨别其中细微的差别,机器能吗?这就是人工智能的下一个巨大挑战:理解语言的细微差别.本文原作者是 Salesforc ...

  5. POJ 2234

    #include<iostream> #include<stdio.h> #include<algorithm> #define MAXN 100 using na ...

  6. css左右布局,左侧固定,右侧自适应

    实现布局的几种方法,见代码: <!DOCTYPE html> <html lang="cn"> <head> <meta charset= ...

  7. vue进行路由拼图的使用案例

    实现思路,利用路由进行实现多个组件拼图: Detail.vue <template> <div> <h1>详细展示</h1> <div>鞍山 ...

  8. Python PIL库学习笔记

    1.PIL简介 Python Imaging Library(缩写为PIL)(在新的版本中被称为Pillow)是Python编程语言的开源库,它增加了对打开,操作和保存许多不同图像文件格式的支持.它适 ...

  9. 最新版chrome浏览器如何离线安装crx插件?(转载)

    原文链接:https://newsn.net/say/chrome-crx-offline.html mac新版chrome开启离线插件安装 对于mac新版chrome,注意,大家一定要按照顺序来.m ...

  10. java获取某段时间内的月份列表

    /**获取两个时间节点之间的月份列表**/ private static List<String> getMonthBetween(String minDate, String maxDa ...