This is the Java solutions of Leetcode problems.
/*
* 17. Letter Combinations of a Phone Number
* Target: Given a string containing digits from 2-9 inclusive, return all possible letter combinations that the number could represent.
* Difficulty：Medium
* Classification：String, Backtracking
*/
/*
* Solution 1
* 2019-06-08 Runtime: 0 ms
* Algorithm: => Recursive
* If digits == "23",
* first layer recursive: combination("a", "23", 1, ""), combination("b", "23", 1, ""), combination("c", "23", 1, "");
* Start with 'a', then recursively to 'd' , then 'g' , add "adg", then add "adh", then add "adi" ... from left to right,
* recursively after it is added.
* Time Complexity: O(logn), Space Conplexity: O(1)
*/
class Solution {
...
}
/*
* Solution 2
* 2019-06-08 Runtime: 0 ms
* Algorithm: => Queue iteration
* If digits == "236", first loop: res = [a, b, c], second loop: res = [b, c, ad, af, ae]...
* Until i == 2, res = [ad, ae, af, bd, be, bf, cd, ce, cf]
* Time Complexity: O(n), Space Conplexity: O(n)
*/
class Solution {
...
}
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