0. 339. Nested List Weight Sum.java Level: Easy Tags: [BFS, DFS, NestedInteger]

给一串integers, list里面可能有nest list. 算总的sum. 规则, 如果是nested list, 每深一个depth, sum要乘以depth.

• New interface to understand: object contains integer or object
  • Visit all && sum, consider dfs.
  • 简单的处理nested structure, dfs增加depth.
• time: visit all nodes eventually, O(n), space O(n)

• bfs, queue, 处理queue.size() for a level
• use a level variable to track levels
• slower since it uses extra space, worst case O(n) of all items

1. 364. Nested List Weight Sum II.java Level: Medium Tags: [DFS, NestedInteger]

• Build a list of NestedInt
• DFS:
  • sum up integers in the list are integers
  • dfs on nested list
  • overallSum = sum * (depth+1)
  • End state: if no nested list (no more child dfs), return depth 1
• Parent level: sum up all ints and times the (depth+1)

• Using stack to flatten all nestedList, and process in the end
• Can actually use list, does not need to be stack.
• uses more memory

2. 341. Flatten Nested List Iterator.java Level: Medium Tags: [Design, NestedInteger, Stack]

• Option1: always set integer on top of the stack everywhere
  • if not, poping stack until the top is integer
  • code is easy
• Option2: in hasNext(), faltten the list in stack

• 用queue to store all items. Kinda hack. Defeat the purpose of the problem.
• Super fast to query next(), however, needs to holds everything in memory
• O(n)