LeetCode-17. 电话号码的字母组合

回溯#

给定一个仅包含数字 2-9 的字符串，返回所有它能表示的字母组合。答案可以按 任意顺序 返回。

给出数字到字母的映射如下（与电话按键相同）。注意 1 不对应任何字母。

示例 1：

1
输入：digits = "23"
2
输出：["ad","ae","af","bd","be","bf","cd","ce","cf"]

示例 2：

1
输入：digits = "2"
2
输出：["a","b","c"]

1
func letterCombinations(digits string) []string {
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    if len(digits) == 0 {
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        return nil
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    }
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    m := []string{"","","abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"}
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    res := []string{}
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    path:= []byte{}
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    var backtrack func(index int)
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    backtrack = func(index int) {
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        if index == len(digits) {
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            res = append(res, string(path))
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            return
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        }
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        digit := digits[index] - '0'
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        letters := m[digit]
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        for i := 0; i < len(letters); i++ {
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            path = append(path, letters[i])
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            backtrack(index+1)
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            path = path[:len(path)-1]
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        }
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    }
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    backtrack(0)
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    return res
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}