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.
下面是代码,但是这里需要注意:temp变量在递归完后必须将递归一次新增的一个字符给去掉。其实我也很有疑问,不是说形参无法传值给实参么?怎么递归后temp值不是保存递归前的值,而是将递归中改变的值返回来了?求大神解释!
const string letters[10] = {" ", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
class Solution
{
public:
vector<string> result;
void aletter(string digits, string temp)
{
if(!digits.length())
{
result.push_back(temp);
}
else
{
int index = digits[0] - '0';
for(int i=0; i<letters[index].length(); i++)
{
temp += letters[index][i];
aletter(digits.substr(1),temp);
temp = temp.substr(0,temp.length()-1);
}
}
}
vector<string> letterCombinations(string digits)
{
result.clear();
aletter(digits, "");
return result;
}
};
另外我还在这里看到了另一种比较简洁的做法,贴上来。
vector<string> letterCombinations(string digits)
{
// Start typing your C/C++ solution below
// DO NOT write int main() function
const string letters[] = {"abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
vector<string> ret(1, "");
for (int i = 0; i < digits.size(); ++i)
{
for (int j = ret.size() - 1; j >= 0; --j)
{
const string &s = letters[digits[i] - '2'];
for (int k = s.size() - 1; k >= 0; --k)
{
if (k)
ret.push_back(ret[j] + s[k]);
else
ret[j] += s[k];
}
}
}
return ret;
}