电话号码的字母组合
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：
输入：digits = "23"
输出：["ad","ae","af","bd","be","bf","cd","ce","cf"]
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/**
* @param {string} digits
* @return {string[]}
*/
var letterCombinations = function(digits) {
if (!digits) return []
let numMap = new Map([
['1', ''],
['2', 'abc'],
['3', 'def'],
['4', 'ghi'],
['5', 'jkl'],
['6', 'mno'],
['7', 'pqrs'],
['8', 'tuv'],
['9', 'wxyz']
])
function helper(digits, output) {
let curNum = digits[0]
let curLetter = numMap.get(curNum)
if (!output.length) {
for (let i = 0; i < curLetter.length; i++) {
output.push(curLetter[i])
}
} else {
let newOutPut = []
for (let m = 0; m < output.length; m++) {
for (let n = 0; n < curLetter.length; n++) {
newOutPut.push(output[m] + curLetter[n])
}
}
output = newOutPut
}
digits = digits.slice(1)
if (digits) {
output = helper(digits, output)
}
return output
}
return helper(digits, [])
};
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来源：力扣（LeetCode） 链接：https://leetcode-cn.com/problems/letter-combinations-of-a-phone-number