leetcode 17 -- Letter Combinations of a Phone Number

Letter Combinations of a Phone Number

题目：

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”].

代码：

//列举出全部可能出现的字符串
string key_val[] = {"0", "1", "abc", "def", "ghi", "jkl", "mno",
"pqrs", "tuv", "wxyz"};
class Solution {
public:
//深度优先搜索，ret是返回结果，dep是字符串的层次（深度），cur_ret是当前的可能结果，digits是目标字符串
void dfs(vector<string> &ret, int dep, string &cur_ret, string digits){
//假设深度已经到达目标字符串末尾，就返回
if(dep == digits.size()){
ret.push_back(cur_ret);
return;
}
//通过深度获取当前字符串
string cur_str = key_val[digits[dep]-'0'];
//对每个字符再进行dfs。
for(int i = 0; i < cur_str.size(); ++i){
cur_ret.push_back(cur_str[i]);
dfs(ret, dep+1, cur_ret, digits);
//这个保证返回时cur_ret还是为空。
cur_ret.pop_back();
}
}

vector<string> letterCombinations(string digits){
vector<string>ret;
//注意推断0的情况
if(digits.size() == 0){
return ret;
}
string cur_re("");
dfs(ret, 0, cur_re, digits);
return ret;
}
};