tree

Tree

144.Binary-Tree-Preorder-Traversal (M+) 145.Binary-Tree-Postorder-Traversal (H-) 270.Closest-Binary-Search-Tree-Value (M+) 095.Unique-Binary-Search-Trees-II (H) 094.Binary Tree Inorder Traversal (H-) 110.Balanced-Binary-Tree (M+) 124.Binary-Tree-Maximum-Path-Sum (M+) 222.Count-Complete-Tree-Nodes (M+) 099.Recover-Binary-Search-Tree (H) 114.Flatten-Binary-Tree-to-Linked-List (M+) 098.Validate-Binary-Search-Tree (M) 117.Populating Next Right Pointers in Each Node II (H) 156.Binary-Tree-Upside-Down (H) 285.Inorder-Successor-in-BST (M) 298.Binary-Tree-Longest-Consecutive Sequence (M+) 450.Delete-Node-in-a-BST (H) 437.Path-Sum-III (H-) 333.Largest-BST-Subtree (H) 543.Diameter-of-Binary-Tree (M+) 572.Subtree-of-Another-Tree (M) 549.Binary-Tree-Longest-Consecutive-Sequence-II (M) 173.Binary-Search-Tree-Iterator (M) 545.Boundary-of-Binary-Tree (H-) 272.Closest-Binary-Search-Tree-Value-II (M+) 310.Minimum-Height-Trees (H-) 226.Invert-Binary-Tree (M) 655.Print-Binary-Tree (M+) 897.Increasing-Order-Search-Tree (M+) 501.Find-Mode-in-Binary-Search-Tree (M+) 558.Quad-Tree-Intersection (M+) 662.Maximum-Width-of-Binary-Tree (H-) 687.Longest-Univalue-Path (M+) 742.Closest-Leaf-in-a-Binary-Tree (H) 834.Sum-of-Distances-in-Tree (H) 863.All-Nodes-Distance-K-in-Binary-Tree (H-) 958.Check-Completeness-of-a-Binary-Tree (M+) 1339. Maximum-Product-of-Splitted-Binary-Tree (TBD) 1104.Path-In-Zigzag-Labelled-Binary-Tree (M+) 1660.Correct-a-Binary-Tree (M+) 1666.Change-the-Root-of-a-Binary-Tree (H-) 1932.Merge-BSTs-to-Create-Single-BST (H) 2003.Smallest-Missing-Genetic-Value-in-Each-Subtree (H)

Serialization & Hashing

297.Serialize-and-Deserialize-Binary-Tree （H-） 652.Find-Duplicate-Subtrees (H) 1948.Delete-Duplicate-Folders-in-System (H)

Traversal

Classic tree level order traversal with O(n) space

bfsQueue = deque()
bfsQueue.append( root )
while bfsQueue:
  head = bfsQueue.popleft()
  // do stuff
  if head.left is not None:
    bfsQueue.append(head.left)
  if head.right is not None:
    bfsQueue.append(head.right)

Special tree level order traversal with O(1) space: example problem (populate next right pointers in each node II)

Get inorder traversal predecessor/successor

    def getPredecessor(root: TreeNode, target: TreeNode) -> TreeNode:
      if target.left:
        currNode = target.left
        while currNode.right:
          currNode = currNode.right
        return currNode
      else:
        predecessor = None
        currNode = root
        while currNode != target:
          if currNode.val >= target.val:
            currNode = currNode.left
          else:
            predecessor = currNode
            currNode = currNode.right
        return predecessor

    def getSuccessor(root: TreeNode, target: TreeNode) -> TreeNode:
      if target.right:
        currNode = target.right
        while currNode.left != null:
          currNode = currNode.left
        return currNode
      else:
        successor = None
        currNode = root
        while currNode != target:
          if currNode.val >= target.val:
            successor = currNode
            currNode = currNode.left
          else:
            currNode = currNode.right
        return successor

Tree & Sequence

105.Construct-Binary-Tree-from-Preorder-and-Inorder-Traversal (H-) 106.Construct-Binary-Tree-from-Inorder-and-Postorder-Traversal (M+) 331.Verify-Preorder-Serialization-of-a-Binary-Tree (H) 449.Serialize-and-Deserialize-BST (H) 971.Flip-Binary-Tree-To-Match-Preorder-Traversal (M+) 1028.Recover-a-Tree-From-Preorder-Traversal (H-) 1569.Number-of-Ways-to-Reorder-Array-to-Get-Same-BST (H) 1597.Build-Binary-Expression-Tree-From-Infix-Expression (H) 1902.Depth-of-BST-Given-Insertion-Order (H-)

LCA

Problem and BF

Generic problem: Find lowest common ancestor for M nodes in a N-ary tree
BF solution: Find root to node paths for M nodes as M list. Then compare M list together to find the first diff element.
- T.C. : build path O(M * SIZE(tree)) and find common LCA O(SIZE(tree) * M)
- S.C. : O(M * SIZE(tree))

Improved solution thoughts

# pseudo code:
# Find the first node whose children + itself covers the entire target during the tree DFS process
(resultContained, resultNode) = recursion(root, [target1, ..., targetN])
  resultContained = 0
  resultNode = None
  for child in root.children:
    childContained, childResult = recursion(child, [target1, ..., targetN])
    if childContained == len(targetSet):
      return (childContained, childResult)
    else:
      resultContained += childContained
  
  if root.value in targetSet:
    resultContained += 1
    if resultContained == len(targetSet):
      resultNode = root
  return (resultContained, resultNode)