# Halo

A magic place for coding

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## beta-recsys v0.1版本发布啦！🎉🎉🎉

我们的beta-recsys v0. 1版本发布了！这是一个基于pytorch 的推荐系统来源平台，我们致力于整合更多的推荐系统模型，为推荐系统这个社区提供一个公平比较的平台，同时推进代码的可复现性！
Github: https://github.com/beta-team/beta-recsys

**teaser video:**https://vimeo.com/456391232

## Problem

Given a string num that contains only digits and an integer target, return all possibilities to add the binary operators '+', '-', or '*' between the digits of num so that the resultant expression evaluates to the target value.

## Problem

You are given an integer array stations that represents the positions of the gas stations on the x-axis. You are also given an integer k.

You should add k new gas stations. You can add the stations anywhere on the x-axis, and not necessarily on an integer position.

Let penalty() be the maximum distance between adjacent gas stations after adding the k new stations.

Return the smallest possible value of penalty(). Answers within 10-6 of the actual answer will be accepted.

## Problem

Given a string s representing a valid expression, implement a basic calculator to evaluate it, and return the result of the evaluation.

Note: You are not allowed to use any built-in function which evaluates strings as mathematical expressions, such as eval().

## Problem

Given an integer array nums, return the number of all the arithmetic subsequences of nums.

A sequence of numbers is called arithmetic if it consists of at least three elements and if the difference between any two consecutive elements is the same.

• For example, [1, 3, 5, 7, 9], [7, 7, 7, 7], and [3, -1, -5, -9] are arithmetic sequences.
• For example, [1, 1, 2, 5, 7] is not an arithmetic sequence.

A subsequence of an array is a sequence that can be formed by removing some elements (possibly none) of the array.

• For example, [2,5,10] is a subsequence of [1,2,1,**2**,4,1,**5**,**10**].

The test cases are generated so that the answer fits in 32-bit integer.

## Problem

You are given an integer n. You have an n x n binary grid grid with all values initially 1‘s except for some indices given in the array mines. The ith element of the array mines is defined as mines[i] = [xi, yi] where grid[xi][yi] == 0.

Return the order of the largest axis-aligned plus sign of 1*’s contained in* grid. If there is none, return 0.

An axis-aligned plus sign of 1‘s of order k has some center grid[r][c] == 1 along with four arms of length k - 1 going up, down, left, and right, and made of 1‘s. Note that there could be 0‘s or 1‘s beyond the arms of the plus sign, only the relevant area of the plus sign is checked for 1‘s.

## Problem

You are given a string s of lowercase English letters and an integer array shifts of the same length.

Call the shift() of a letter, the next letter in the alphabet, (wrapping around so that 'z' becomes 'a').

• For example, shift('a') = 'b', shift('t') = 'u', and shift('z') = 'a'.

Now for each shifts[i] = x, we want to shift the first i + 1 letters of s, x times.

Return the final string after all such shifts to s are applied.

## Problem

Design a data structure that follows the constraints of a Least Recently Used (LRU) cache.

Implement the LRUCache class:

• LRUCache(int capacity) Initialize the LRU cache with positive size capacity.
• int get(int key) Return the value of the key if the key exists, otherwise return -1.
• void put(int key, int value) Update the value of the key if the key exists. Otherwise, add the key-value pair to the cache. If the number of keys exceeds the capacity from this operation, evict the least recently used key.

The functions get and put must each run in O(1) average time complexity.