1. 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"].
Note:
Although the above answer is in lexicographical order, your answer could be in any order you want.
2. 思路
遍历组合即可,递归处理。
3. 代码
耗时:3ms
class Solution {
public:
vector<string> letterCombinations(string digits) {
vector<string> ret;
for (int i = 0; i < digits.size(); i++) {
if (digits[i] < '2' || digits[i] > '9') return ret;
}
if (digits.length() == 0) {
return ret;
} else {
int k = digits[0] - '0';
for (int i = 0; i < lens[k]; i++) {
char ch = m[k][i];
vector<string> subs = letterCombinations(digits.substr(1));
if (subs.size() == 0) {
string s;
s += ch;
ret.push_back(s);
} else {
for (int j = 0; j < subs.size(); j++) {
string s;
s += ch;
s += subs[j];
ret.push_back(s);
}
}
}
}
return ret;
}
private:
static const int lens[10];
static const char m[10][4];
};
const int Solution::lens[10] = {0, 0, 3, 3, 3, 3, 3, 4, 3, 4};
const char Solution::m[10][4] = {{0, 0, 0, 0}, {0, 0, 0, 0}, {'a', 'b', 'c', 0}, {'d', 'e', 'f', 0},
{'g', 'h', 'i', 0}, {'j', 'k', 'l', 0}, {'m', 'n', 'o', 0},
{'p', 'q', 'r', 's'}, {'t', 'u', 'v', 'w'}, {'w', 'x', 'y', 'z'}};
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