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package leetcode._17_;

import java.util.ArrayList;

import java.util.Arrays;

import java.util.Collections;

import java.util.List;

/**

* Created by zhangbo54 on 2019-03-05.

*/

class Solution {

public List<String> letterCombinations(String digits) {

if (digits.length() == 0) {

return Collections.emptyList();

}

if (digits.length() == 1) {

return singleDigitToStringList(digits.charAt(0));

}

List<String> leftArr = this.letterCombinations(digits.substring(0, digits.length() / 2));

List<String> rightArr = this.letterCombinations(digits.substring(digits.length() / 2));

List<String> result = new ArrayList<>();

for (String left : leftArr) {

for (String right : rightArr) {

result.add(left + right);

}

}

return result;

}

private List<String> singleDigitToStringList(char digit) {

switch (digit) {

case '2':

return Arrays.asList("a", "b", "c");

case '3':

return Arrays.asList("d", "e", "f");

case '4':

return Arrays.asList("g", "h", "i");

case '5':

return Arrays.asList("j", "k", "l");

case '6':

return Arrays.asList("m", "n", "o");

case '7':

return Arrays.asList("p", "q", "r", "s");

case '8':

return Arrays.asList("t", "u", "v");

case '9':

return Arrays.asList("w", "x", "y", "z");

default:

return Arrays.asList("");

}

}

}