Added descriptions 17-19.

src/main/java/g0001_0100/s0017_letter_combinations_of_a_phone_number/readme.md

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+17\. Letter Combinations of a Phone Number
+
+Medium
+
+Given a string containing digits from `2-9` inclusive, return all possible letter combinations that the number could represent. Return the answer in **any order**.
+
+A mapping of digit to letters (just like on the telephone buttons) is given below. Note that 1 does not map to any letters.
+
+![](https://upload.wikimedia.org/wikipedia/commons/thumb/7/73/Telephone-keypad2.svg/200px-Telephone-keypad2.svg.png)
+
+**Example 1:**
+
+**Input:** digits = "23"
+
+**Output:** \["ad","ae","af","bd","be","bf","cd","ce","cf"\] 
+
+**Example 2:**
+
+**Input:** digits = ""
+
+**Output:** \[\] 
+
+**Example 3:**
+
+**Input:** digits = "2"
+
+**Output:** \["a","b","c"\] 
+
+**Constraints:**
+
+*   `0 <= digits.length <= 4`
+*   `digits[i]` is a digit in the range `['2', '9']`.

src/main/java/g0001_0100/s0018_four_sum/readme.md

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+18\. 4Sum
+
+Medium
+
+Given an array `nums` of `n` integers, return _an array of all the **unique** quadruplets_ `[nums[a], nums[b], nums[c], nums[d]]` such that:
+
+*   `0 <= a, b, c, d < n`
+*   `a`, `b`, `c`, and `d` are **distinct**.
+*   `nums[a] + nums[b] + nums[c] + nums[d] == target`
+
+You may return the answer in **any order**.
+
+**Example 1:**
+
+**Input:** nums = \[1,0,-1,0,-2,2\], target = 0
+
+**Output:** \[\[-2,-1,1,2\],\[-2,0,0,2\],\[-1,0,0,1\]\] 
+
+**Example 2:**
+
+**Input:** nums = \[2,2,2,2,2\], target = 8
+
+**Output:** \[\[2,2,2,2\]\] 
+
+**Constraints:**
+
+*   `1 <= nums.length <= 200`
+*   `-109 <= nums[i] <= 109`
+*   `-109 <= target <= 109`

src/main/java/g0001_0100/s0019_remove_nth_node_from_end_of_list/readme.md

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+19\. Remove Nth Node From End of List
+
+Medium
+
+Given the `head` of a linked list, remove the `nth` node from the end of the list and return its head.
+
+**Example 1:**
+
+![](https://assets.leetcode.com/uploads/2020/10/03/remove_ex1.jpg)
+
+**Input:** head = \[1,2,3,4,5\], n = 2
+
+**Output:** \[1,2,3,5\] 
+
+**Example 2:**
+
+**Input:** head = \[1\], n = 1
+
+**Output:** \[\] 
+
+**Example 3:**
+
+**Input:** head = \[1,2\], n = 1
+
+**Output:** \[1\] 
+
+**Constraints:**
+
+*   The number of nodes in the list is `sz`.
+*   `1 <= sz <= 30`
+*   `0 <= Node.val <= 100`
+*   `1 <= n <= sz`
+
+**Follow up:** Could you do this in one pass?