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‎en/1-1000/1-two-sum.md‎

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LeetCode link: [1. Two Sum](https://leetcode.com/problems/two-sum), difficulty: **Easy**
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## LeetCode description of "1. Two Sum"
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Given an array of integers `nums` and an integer `target`, return _indices of the two numbers such that they add up to `target`_.
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Given an array of integers `nums` and an integer `target`, return *indices of the two numbers such that they add up to `target`*.
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You may assume that each input would have **_exactly_ one solution**, and you may not use the same element twice.
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You may assume that each input would have ***exactly* one solution**, and you may not use the same element twice.
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You can return the answer in any order.
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**Output**: `[1,2]`
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### [Constraints]
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- `2 <= nums.length <= 10**4`
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- `-10**9 <= nums[i] <= 10**9`
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- `-10**9 <= target <= 10**9`
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- `2 <= nums.length <= 10^4`
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- `-10^9 <= nums[i] <= 10^9`
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- `-10^9 <= target <= 10^9`
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- **Only one valid answer exists.**
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### [Hints]
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### Steps
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1. In `Map`, `key` is `num`, and `value` is array `index`.
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```javascript
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let numToIndex = new Map()
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for (let i = 0; i < nums.length; i++) {
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numToIndex.set(nums[i], i)
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}
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```
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```javascript
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let numToIndex = new Map()
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for (let i = 0; i < nums.length; i++) {
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numToIndex.set(nums[i], i)
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}
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```
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2. Traverse the array, if `target - num` is in `Map`, return it. Otherwise, add `num` to `Map`.
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```javascript
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let numToIndex = new Map()
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for (let i = 0; i < nums.length; i++) {
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if (numToIndex.has(target - nums[i])) { // 1
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return [numToIndex.get(target - nums[i]), i] // 2
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}
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numToIndex.set(nums[i], i)
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}
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```
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```javascript
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let numToIndex = new Map()
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for (let i = 0; i < nums.length; i++) {
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if (numToIndex.has(target - nums[i])) { // 1
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return [numToIndex.get(target - nums[i]), i] // 2
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}
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numToIndex.set(nums[i], i)
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}
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```
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### Complexity
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* Time: `O(n)`.

‎en/1-1000/15-3sum.md‎

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### [Constraints]
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- `3 <= nums.length <= 3000`
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- `-10**5 <= nums[i] <= 10**5`
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- `-10^5 <= nums[i] <= 10^5`
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### [Hints]
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<details>
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<summary>Hint 1</summary>
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So, we essentially need to find three numbers x, y, and z such that they add up to the given value. If we fix one of the numbers say x, we are left with the two-sum problem at hand!
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So, we essentially need to find three numbers `x`, `y`, and `z` such that they add up to the given value. If we fix one of the numbers say `x`, we are left with the two-sum problem at hand!
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</details>
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<details>
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<summary>Hint 2</summary>
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For the two-sum problem, if we fix one of the numbers, say x, we have to scan the entire array to find the next number y, which is value - x where value is the input parameter. Can we change our array somehow so that this search becomes faster?
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For the two-sum problem, if we fix one of the numbers, say `x`, we have to scan the entire array to find the next number `y`, which is `value - x` where value is the input parameter. Can we change our array somehow so that this search becomes faster?
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</details>
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<details>
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</details>
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## Intuition
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1. The `sum` of three numbers equals `0`, which is equivalent to the `sum` of _two numbers_ equaling the _**negative** third number_.
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1. The `sum` of three numbers equals `0`, which is equivalent to the `sum` of *two numbers* equaling the ***negative** third number*.
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2. There are two options:
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1. First `determine two numbers` and `find the third number`
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2. First `determine one number` and `find the other two numbers`
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5. For `option 1`, only the `Python` sample code is given. This article focuses on `option 2`.
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## Steps
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1. Sort `nums`.
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2. Iterate over `nums`.
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3. pseudocode:
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```javascript
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for (i = 0; i < nums.length; i++) {
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left = i + 1
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right = nums.length - 1
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while (left < right) {
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if (condition1) {
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left += 1
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} else (condition2) {
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right -= 1
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}
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}
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}
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```
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```javascript
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for (i = 0; i < nums.length; i++) {
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left = i + 1
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right = nums.length - 1
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while (left < right) {
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if (condition1) {
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left += 1
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} else (condition2) {
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right -= 1
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}
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}
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}
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```
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## Complexity
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* Time: `O(N * N)`.
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* Space: `O(N)`.
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## Python
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### Solution 1: Use Map (complex and slow)
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```python
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# from collections import defaultdict
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‎en/1-1000/18-4sum.md‎

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LeetCode link: [18. 4Sum](https://leetcode.com/problems/4sum), difficulty: **Medium**
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## LeetCode description of "18. 4Sum"
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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:
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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:
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* `0 <= a, b, c, d < n`
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* `a`, `b`, `c`, and `d` are **distinct**.
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* `nums[a] + nums[b] + nums[c] + nums[d] == target`
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* You may return the answer in **any order**.
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- `0 <= a, b, c, d < n`
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- `a`, `b`, `c`, and `d` are **distinct**.
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- `nums[a] + nums[b] + nums[c] + nums[d] == target`
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- You may return the answer in **any order**.
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### [Example 1]
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**Input**: `nums = [1,0,-1,0,-2,2], target = 0`
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### [Constraints]
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- `1 <= nums.length <= 200`
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- `-10**9 <= nums[i] <= 10**9`
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- `-10**9 <= target <= 10**9`
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- `-10^9 <= nums[i] <= 10^9`
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- `-10^9 <= target <= 10^9`
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## Intuition
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1. The idea of this question is the same as [15. 3Sum](15-3sum.md), please click the link to view.
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4. You may have already seen that no matter it is `two numbers`, `three numbers` or `four numbers`, the `Two Pointers Technique` can be used.
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## Complexity
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* Time: `O(N**3)`.
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* Time: `O(N^3)`.
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* Space: `O(N)`.
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## Python

‎en/1-1000/19-remove-nth-node-from-end-of-list.md‎

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## Steps
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1. First find out `node_count`.
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```ruby
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node_count = 0
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node = head
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while node
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node_count += 1
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node = node.next
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end
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```
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```ruby
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node_count = 0
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node = head
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while node
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node_count += 1
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node = node.next
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end
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```
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2. When `index == node_count - N`, delete the node by `node.next = node.next.next`.
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```ruby
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index = 0
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node = head
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while node
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if index == node_count - n
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node.next = node.next.next
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break
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end
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index += 1
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node = node.next
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end
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```
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```ruby
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index = 0
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node = head
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while node
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if index == node_count - n
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node.next = node.next.next
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break
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end
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index += 1
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node = node.next
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end
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```
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3. Since the deleted node may be `head`, a virtual node `dummy_node` is used to facilitate unified processing.
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```ruby
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dummy_head = ListNode.new # 1
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dummy_head.next = head # 2
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node = dummy_head # 3
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# omitted code
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return dummy_head.next
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```
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```ruby
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dummy_head = ListNode.new # 1
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dummy_head.next = head # 2
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node = dummy_head # 3
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# omitted code
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return dummy_head.next
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```
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## Complexity
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* Time: `O(N)`.

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