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// @Title: 电话号码的字母组合 (Letter Combinations of a Phone Number)
// @Author: 15816537946@163.com
// @Date: 2020-10-30 09:14:48
// @Runtime: 0 ms
// @Memory: 2 MB
func letterCombinations(digits string) []string {
	if len(digits) == 0 {
		return nil
	}

	phoneMap := map[int][]string{
		2: []string{
			"a", "b", "c",
		},
		3: []string{
			"d", "e", "f",
		},
		4: []string{
			"g", "h", "i",
		},
		5: []string{
			"j", "k", "l",
		},
		6: []string{
			"m", "n", "o",
		},
		7: []string{
			"p", "q", "r", "s",
		},
		8: []string{
			"t", "u", "v",
		},
		9: []string{
			"w", "x", "y", "z",
		},
	}

	ans := make([]string, 0)
	var backtrack func(string, []byte)
	backtrack = func(out string, idx []byte) {
		if len(idx) == 0 {
			tmp := out
			ans = append(ans, tmp)
			return
		}
		for _, letters := range phoneMap[int(idx[0]-'0')] {
			for _, letter := range letters {
				backtrack(out+string(letter), idx[1:])
			}
		}
	}

	backtrack("", []byte(digits))
	return ans

}