This repository contains all solved problems, only med+ problems are addressed briefly how they are done. Readability is not considered when coding.

• LRUCache https://leetcode.com/problems/lru-cache/
  Inorder to meet with O(1) time complexity, a double linked list and a node array are used. Demonstrated as below.

  class Node {
        Node prev;
        Node next;
        int key;
        int val;
  }
  Node[] nodes = new Node[10001];

  The key idea is to maintain the list properly. When a put(int key, int value) occurs, the node with the key will be added, or moved if existed, to the list tail. When a get(int key) operation comes, node[key] enables us to find the node quickly, we then move the found node to the list tail. Not only the list operation but the node finding are with O(1), hence O(1) satisfies.

• Word Search https://leetcode.com/problems/word-search/description/
  Iterate all board characters, when a character is found equal to the word's first character, do a BFS.

• Count vowel strings in ranges https://leetcode.com/problems/count-vowel-strings-in-ranges/description/
  Iterate through the string array with an index i, and use a counter array to sum up the number of vowel string at the index.

• Zigzag Conversion https://leetcode.com/problems/zigzag-conversion/description/
  It's all about array loop control, not too complex.

• Peeking Iterator https://leetcode.com/problems/peeking-iterator/description/
  Much more of a design problem.