0. 938. Range Sum of BST.java Level: Easy Tags: [BST, Recursion, Tree]

• sum up the matching L & R
  • Find (L,R) on left child
  • Find (L,R) on right child
  • Find (L,R) covering the root node
• space O(n), worst case O(logn), height of dfs.
• time O(n) to find all nodes between (L, R)

• Using stack, or queue, list: any data structure (we are not doing ordered search)
• space O(n)
• time O(n)

1. 698. Partition to K Equal Sum Subsets.java Level: Medium Tags: [DFS, DP, Recursion]

• Target = total / k, fixed.
• DFS: Pick one num and dfs with the remaining nums for subproblem
  • subproblem1: accumulate local sum to target
    • EndState: accumulatedSum == target, continue with below
  • subproblem2: after accumulatedSum == target, continue dfs with k-1
    • EndState: k == 0, return overall true
• Option1: DFS with nums, and boolean[] visited. Fast
• Option2: DFS with queue.
  • Specially handling: dfs(size+1) to prevent while(size-- >0) from skipping last item at index 0

• the problem aims to find true/false capability
• bit-masking. TODO. The DP approach is kinda hard-level
• https://leetcode.com/problems/partition-to-k-equal-sum-subsets/discuss/335668/DP-with-Bit-Masking-Solution-%3A-Best-for-Interviews