1. Merge k Sorted Lists
2. Critical Connections in a Network
3. Integer to English Words
4. Alien Dictionary
5. Serialize and Deserialize Binary Tree
6. Word Break II
Tips 总结:
1. String + Math 的题目常用到 Stack,而且有许多 edge cases;
2. Alien Dictionary 简直好题,topological sort;
3. Preorder traversal with null can be used to serialize and deserialize a binary tree;
4. 如果一道 Backtracking 题 TLE,可以考虑加个 memo,变成 top-down dp;