|17|[Letter Combinations of a Phone Number](https://leetcode.com/problems/letter-combinations-of-a-phone-number/)|[Solution](../../blob/master/MEDIUM/src/medium/LetterCombinationsofaPhoneNumber.java)|O(n*4^n)|O(n)|Medium|Backtracking

|15|[3Sum](https://leetcode.com/problems/3sum/)|[Solution](../../blob/master/MEDIUM/src/medium/_3Sum.java)|O(n^2)|O(1)|Medium|Two Pointers

|14|[Longest Common Prefix](https://leetcode.com/problems/longest-common-prefix/)|[Solution](../../blob/master/EASY/src/easy/LongestCommonPrefix.java)| O(n*min(wordLength in this array)) | O(1) | Easy

|12|[Integer to Roman](https://leetcode.com/problems/integer-to-roman/)|[Solution](../../blob/master/MEDIUM/src/medium/IntegertoRoman.java)|O(1)|O(1)|Medium|

|10|[Regular Expression Matching](https://leetcode.com/problems/regular-expression-matching/)|[Solution](../../blob/master/HARD/src/hard/RegularExpressionMatching.java)|O(m*n)|O(m*n)|Hard|DP

|9|[Palindrome Number](https://leetcode.com/problems/palindrome-number/)|[Solution](../../blob/master/EASY/src/easy/PalindromeNumber.java)| O(logn)/(n) | O(1) | Easy