The post summarizes high-frequency coding problems reported for Apple Software Engineer interviews. Practice the following algorithm and data-structure questions: 1. **Design a recency-based cache**: Implement a fixed-capacity cache with `get(key)` and `put(key, value)` operations. Both operations should run in **O(1)** average time. When the cache is full, evict the **least recently used** item. 2. **Design a set with random retrieval**: Build a data structure that supports `insert(val)`, `remove(val)`, and `getRandom()`, where each operation runs in **O(1)** average time. 3. **Build a prefix word dictionary**: Implement a structure with `insert(word)`, `search(word)`, and `startsWith(prefix)`. 4. **Return the k most frequent values**: Given an integer array such as `nums = [1,1,1,2,2,3]` and `k = 2`, return the `k` values that appear most often, for example `[1,2]` in any order. 5. **Compute product except self**: Given an array like `[1,2,3,4]`, return an array where each position contains the product of all other elements, for example `[24,12,8,6]`. Do not use division, and aim for **O(n)** time. 6. **Measure trapped water between bars**: Given a non-negative integer array representing elevation heights, for example `[0,1,0,2,1,0,1,3,2,1,2,1]`, compute how much rainwater can be trapped. 7. **Find the largest water container**: Given heights of vertical lines, choose two lines that together with the x-axis hold the maximum amount of water, and return that maximum area. 8. **Find the longest substring without duplicates**: Given a string such as `"abcabcbb"`, return the length of the longest substring that contains no repeated characters. For this example, the answer is `3`. 9. **Group words by letter composition**: Given a list like `["eat","tea","tan","ate","nat","bat"]`, group together words that are made of the same letters, for example `[["eat","tea","ate"],["tan","nat"],["bat"]]`. 10. **Decode nested repetition strings**: Given an encoded string such as `"3[a2[c]]"`, return its decoded form. For this example, the result is `"accaccacc"`. 11. **Count islands in a grid**: Given a 2D grid of land and water cells, count how many disconnected islands exist using BFS or DFS. 12. **Check whether all courses can be completed**: Given `n` courses and a list of prerequisite pairs, determine whether it is possible to finish all courses. This is a graph cycle-detection / topological-order problem. 13. **Merge multiple sorted linked lists**: Given `k` sorted linked lists such as `1->4->5`, `1->3->4`, and `2->6`, merge them into one sorted linked list: `1->1->2->3->4->4->5->6`. 14. **Rotate a square matrix clockwise**: Given an `n x n` matrix, rotate it **90 degrees clockwise in place**. Example: `[[1,2,3],[4,5,6],[7,8,9]]` becomes `[[7,4,1],[8,5,2],[9,6,3]]`. 15. **Find the longest mountain subarray**: Given an array such as `[2,1,4,7,3,2,5]`, return the length of the longest contiguous subarray that strictly increases and then strictly decreases. In this example, the longest mountain is `[1,4,7,3,2]`, so the answer is `5`.