17. Letter Combinations of a Phone Number

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每日一题 2019 - 03 - 07

题目：

Given a string containing digits from 2-9 inclusive, 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. Note that 1 does not map to any letters.

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Example:

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Input: "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.

解法：

这个题让我们找到给定电话号码上的所有不重复的排列组合，具体可以用三重循环的方法做，这三重循环的作用分别是：

  • 第一层遍历给定的string digits，因为我们要找到所有的待遍历电话数字
  • 第二层遍历待返回vector<string>，保证digits中的所有数据都能被遍历到，同时在遍历时候由于不能返回过程字符，所以要删除过程中遍历过的量
  • 第三层遍历digts中对应的每一个字母

代码：

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class Solution {
    public:
        vector<string> letterCombinations(string digits) {
            vector<string> res;
            if (digits.empty()) return res;
            string dict[] = {"abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
            res.push_back("");
            for (int i = 0; i < digits.size(); ++i) {
                int n = res.size();
                string str = dict[digits[i] - '2'];
                for (int j = 0; j < n; ++j) {
                    string tmp = res.front();
                    res.erase(res.begin());
                    for (int k = 0; k < str.size(); ++k) {
                        res.push_back(tmp + str[k]);
                    }
                }
            }
            return res;
        }
};
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