17. Letter Combinations of a Phone Number
Given a string containing digits from 2-9 inclusive, return all possible letter combinations that the number could represent. Return the answer in any order.
A mapping of digits to letters (just like on the telephone buttons) is given below. Note that 1 does not map to any letters.
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Example 1:
Input: digits = "23"
Output: ["ad","ae","af","bd","be","bf","cd","ce","cf"]
Example 2:
Input: digits = ""
Output: []
Example 3:
Input: digits = "2"
Output: ["a","b","c"]
C++ Solution
一开始用的是Recursive的方法，直接LeetCode给我报Stack Overflow了，改为使用迭代的方式：
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string numberToLetter(int num) {
switch (num) {
case 2:
return "abc";
case 3:
return "def";
case 4:
return "ghi";
case 5:
return "jkl";
case 6:
return "mno";
case 7:
return "pqrs";
case 8:
return "tuv";
case 9:
return "wxyz";
}
return "";
}
vector<string> letterCombinations(string digits) {
if (digits.empty()) {
return vector<string>();
}
vector<string> result{""};
for (char digit : digits) {
string letters = numberToLetter(digit - '0');
if (letters.empty()) {
continue;
}
vector<string> newResult;
for (char letter : letters) {
for (string str : result) {
newResult.push_back(str + letter);
}
}
result = newResult;
}
return result;
}
