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17.电话号码的字母组合 (opens new window)

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• algorithms
• Medium (59.18%)
• Likes: 2775
• Dislikes: -
• Total Accepted: 830.9K
• Total Submissions: 1.4M
• Testcase Example: '"23"'

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：

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

示例 2：

输入：digits = ""
输出：[]

示例 3：

输入：digits = "2"
输出：["a","b","c"]

 

提示：

• 0 <= digits.length <= 4
• digits[i] 是范围 ['2', '9'] 的一个数字。

/*
 * @lc app=leetcode.cn id=17 lang=javascript
 *
 * [17] 电话号码的字母组合
 * 递归
 */

// @lc code=start
/**
 * @param {string} digits
 * @return {string[]}
 */
var letterCombinations = function(digits) {
    if (digits === '') {
        return []
    }
    const map = {
        2: ['a', 'b', 'c'],
        3: ['d', 'e', 'f'],
        4: ['g', 'h', 'i'],
        5: ['j', 'k', 'l'],
        6: ['m', 'n', 'o'],
        7: ['p', 'q', 'r', 's'],
        8: ['t', 'u', 'v'],
        9: ['w', 'x', 'y', 'z'],
    }

    if (digits.length === 1) {
        return map[digits[0]]
    } else {
        const subResult = letterCombinations(digits.slice(0, digits.length - 1))

        const lastLetterMap = map[digits[digits.length - 1]]
        return subResult.reduce((acc, item) => {
            for (let lastLetter of lastLetterMap) {
                acc.push(`${item}${lastLetter}`)
            }
            return acc
        }, [])
    }
}
// @lc code=end

/*
 * @lc app=leetcode.cn id=17 lang=javascript
 *
 * [17] 电话号码的字母组合
 * 非递归
 */

// @lc code=start
/**
 * @param {string} digits
 * @return {string[]}
 */
var letterCombinations = function(digits) {
    if (digits === '') {
        return []
    }
    const map = {
        2: ['a', 'b', 'c'],
        3: ['d', 'e', 'f'],
        4: ['g', 'h', 'i'],
        5: ['j', 'k', 'l'],
        6: ['m', 'n', 'o'],
        7: ['p', 'q', 'r', 's'],
        8: ['t', 'u', 'v'],
        9: ['w', 'x', 'y', 'z'],
    }

    let result = map[digits[0]]

    for (let i = 1; i < digits.length; i++) {
        const lastLetterMap = map[digits[i]]

        result = result.reduce((acc, item) => {
            for (let lastLetter of lastLetterMap) {
                acc.push(`${item}${lastLetter}`)
            }
            return acc
        }, [])
    }

    return result
}
// @lc code=end