vault backup: 2022-09-02 14:45:32

OJ notes/pages/Leetcode Design-Hash.md

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+# Leetcode Design-Hash
+
+#### 2022-09-01 14:44
+
+> ##### Data structures:
+> #DS #array #linked_list 
+> ##### Difficulty:
+> #coding_problem #difficulty-easy
+> ##### Additional tags:
+> #leetcode #CS_list_need_practicing 
+> ##### Revisions:
+> N/A
+
+##### Links:
+- [Link to problem](https://leetcode.com/problems/design-hashmap/)
+- [Explanation](https://leetcode.com/problems/design-hashmap/discuss/1097755/JS-Python-Java-C%2B%2B-or-(Updated)-Hash-and-Array-Solutions-w-Explanation)
+___
+### Problem
+
+Design a HashMap without using any built-in hash table libraries.
+
+Implement the `MyHashMap` class:
+
+-   `MyHashMap()` initializes the object with an empty map.
+-   `void put(int key, int value)` inserts a `(key, value)` pair into the HashMap. If the `key` already exists in the map, update the corresponding `value`.
+-   `int get(int key)` returns the `value` to which the specified `key` is mapped, or `-1` if this map contains no mapping for the `key`.
+-   `void remove(key)` removes the `key` and its corresponding `value` if the map contains the mapping for the `key`.
+
+#### Examples
+
+**Example 1:**
+
+**Input**
+["MyHashMap", "put", "put", "get", "get", "put", "get", "remove", "get"]
+[[], [1, 1], [2, 2], [1], [3], [2, 1], [2], [2], [2]]
+**Output**
+[null, null, null, 1, -1, null, 1, null, -1]
+
+**Explanation**
+MyHashMap myHashMap = new MyHashMap();
+myHashMap.put(1, 1); // The map is now [[1,1]]
+myHashMap.put(2, 2); // The map is now [[1,1], [2,2]]
+myHashMap.get(1);    // return 1, The map is now [[1,1], [2,2]]
+myHashMap.get(3);    // return -1 (i.e., not found), The map is now [[1,1], [2,2]]
+myHashMap.put(2, 1); // The map is now [[1,1], [2,1]] (i.e., update the existing value)
+myHashMap.get(2);    // return 1, The map is now [[1,1], [2,1]]
+myHashMap.remove(2); // remove the mapping for 2, The map is now [[1,1]]
+myHashMap.get(2);    // return -1 (i.e., not found), The map is now [[1,1]]
+
+#### Constraints
+
+-   `0 <= key, value <= 106`
+-   At most `104` calls will be made to `put`, `get`, and `remove`.
+
+### Thoughts
+
+> [!summary]
+> This is a #array problem.
+
+#### Easy way:
+
+Using array, since 10^6 size is not too large
+
+#### Hard way:
+
+Using hashes with linked list to support collision check.
+
+### Solution
+
+Easy way: 
+
+```cpp
+class MyHashMap {
+  int myMap[1000001];
+
+public:
+  MyHashMap() { fill(myMap, myMap + 1000001, -1); }
+
+  void put(int key, int value) { myMap[key] = value; }
+
+  int get(int key) { return myMap[key]; }
+
+  void remove(int key) { myMap[key] = -1; }
+};
+
+/**
+ * Your MyHashMap object will be instantiated and called as such:
+ * MyHashMap* obj = new MyHashMap();
+ * obj->put(key,value);
+ * int param_2 = obj->get(key);
+ * obj->remove(key);
+ */
+```

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