## trees
* a tree is a widely used abstract data type that represents a hierarchical structure with a set of connected nodes.
* each node in the tree can be connected to many children, but must be connect to exactly one parent (except for the root node).
* a tree is an undirected and connected acyclic graph and there are no cycle or loops.
---
### binary trees
* **binary trees** are trees that have each up to 2 children.
* access, search, remove, insert are all `O(log(N)`. space complexity of traversing balanced trees is `O(h)` where `h` is the height of the tree (while very skewed trees will be `O(N)`.
* the **width** is the number of nodes in a level.
* the **degree** is the nunber of children of a node.
* a **complete tree** is a tree on which every level is fully filled (except perhaps for the last).
* a **perfect tree** is both full and complete (it must have exactly `2**k - 1` nodes, where `k` is the number of levels).
---
### full trees
* a **full binary tree** has each node with either zero or two children (and no node has only one child).
```python
def is_full(node) -> bool:
if node is None:
return True
return bool(node.right and node.left) and is_full(node.right) and is_full(node.left)
```
---
### is leaf?
* a node is called **leaf** if it has no children.
```python
def is_leaf(node):
return bool(not node.right and not node.left)
```
---
### balanced trees
* a **balanced tree** is a binary tree in which the left and right subtrees of every node differ in height by no more than 1.
```python
def height(root):
if root is None:
return -1
return 1 + max(height(root.left), height(root.right))
def is_balanced(root):
if root is None:
return True
return abs(height(root.left) - height(root.right)) < 2 and \
is_balanced(root.left) and is_balanced(root.right)
```
----
### depth of a binary tree
* the **depth** (or level) of node is the number of edges from the tree's root node until the node.
```python
def max_depth(root) -> int:
if root is None:
return 0
return max(max_depth(root.left) + 1, max_depth(root.right) + 1)
```
---
### height of a tree
* the **height** of a node is the number of edges on the **longest path** between that node and a leaf.
* the **height of tree** is the height of its root node, or the depth of its deepest node.
```python
def height(root):
if root is none:
return 0
return 1 + max(height(root.left), height(root.right))
```
---
### tree traversal: breath-first search (level-order)
* give you all elements **in order** with time `O(log(N)`. used to traverse a tree by level.
* iterative solutions use a queue for traversal or find the shortest path from the root node to the target node:
- in the first round, we process the root node.
- in the second round, we process the nodes next to the root node.
- in the third round, we process the nodes which are two steps from the root node, etc.
- newly-added nodes will not be traversed immediately but will be processed in the next round.
- if node X is added to the kth round queue, the shortest path between the root node and X is exactly k.
- the processing order of the nodes in the exact same order as how they were added to the queue (which is FIFO).
```python
def bfs_iterative(root):
result = []
queue = collections.deque([root])
while queue:
node = queue.popleft()
if node:
result.append(node.val)
queue.append(node.left)
queue.append(node.right)
return result
```
---
### tree traversal: depth-first search
- deep-first search (DFS) can also be used to find the path from the root node to the target node if you want to visit every node and/or search the deepest paths firsts.
- recursion solutions are easier to implement; however, if the recursion depth is too high, stack overflow might occur. in that case, you might use BFS instead or implement DFS using an explicit stack (i.e., use a while loop and a stack structure to simulate the system call stack).
- overall, we only trace back and try another path after we reach the deepest node. as a result, the first path you find in DFS is not always the shortest path:
- we first push the root node to the stack, then we try the first neighbor and push its node to the stack, etc.
- when we reach the deepest node, we need to trace back.
- when we track back, we pop the deepest node from the stack, which is actually the last node pushed to the stack.
- the processing order of the nodes is exactly the opposite order of how they are added to the stack.
---
#### in-order
- `left -> node -> right`
- in a bst, in-order traversal will be sorted in the ascending order (therefore, it's the most frequently used method).
- converting a sorted array to a bst with inorder has no unique solution (in another hadnd, both preorder and postorder are unique identifiers of a bst).
```python
def inorder(root):
if root is None:
return []
return inorder(root.left) + [root.val] + inorder(root.right)
def inorder_iterative(root) -> list:
result = []
stack = []
node = root
while stack or node:
if node:
stack.append(node)
node = node.left
else:
node = stack.pop()
result.append(node.val)
node = node.right
return result
```
* we can also build an interator:
```python
class BST_Iterator:
def __init__(self, root):
self.stack = []
self.left_inorder(root)
def left_inorder(self, root):
while root:
self.stack.append(root)
root = root.left
def next(self) -> int:
top_node = self.stack.pop()
if top_node.right:
self.left_inorder(top_node.right)
return top_node.val
def has_next(self) -> bool:
return len(self.stack) > 0
```
---
#### pre-order
- `node -> left -> right`
- top-down (parameters are passed down to children), so deserialize with a queue.
```python
def preorder_recursive(root):
if root is None:
return []
return [root.val] + preorder(root.left) + preorder(root.right)
def preorder_iterative(root) -> list:
result = []
stack = [root]
while stack:
node = stack.pop()
if node:
result.append(node.val)
stack.append(node.right) # not the order (stacks are fifo)
stack.append(node.left)
return result
```
---
#### post-order
- `left -> right -> node`
- bottom-up solution.
- deletion process is always post-order: when you delete a node, you will delete its left child and its right child before you delete the node itself.
- post-order can be used in mathematical expressions as it's easier to write a program to parse a post-order expression. using a stack, each time when you meet an operator, you can just pop 2 elements from the stack, calculate the result and push the result back into the stack.
```python
def postorder(root):
if root is None:
return []
return postorder(root.left) + postorder(root.right) + [root.val]
def postorder_iterative(root) -> list:
stack, result = [], []
node = root
while node or stack:
while node:
if node.right:
stack.append(node.right)
stack.append(node)
node = node.left
node = stack.pop()
if stack and node.right == stack[-1]:
stack[-1] = node
node = node.right
else:
result.append(node.val)
node = None
return result
```
----
### is same tree?
```python
def is_same_trees(p, q):
if not p and not q:
return True
if (not p and q) or (not q and p):
return False
if p.val != q.val:
return False
return is_same_trees(p.right, q.right) and is_same_trees(p.left, q.left)
````
---
### is symmetric?
```python
def is_symmetric(root) -> bool:
stack = [(root, root)]
while stack:
node1, node2 = stack.pop()
if (not node1 and node2) or (not node2 and node1):
return False
if node1 and node2:
if node1.val != node2.val:
return False
stack.append([node1.left, node2.right])
stack.append([node1.right, node2.left])
return True
def is_symmetric_recursive(root) -> bool:
def helper(node1, node2):
if (not node1 and node2) or \
(not node2 and node1) or \
(node1 and node2 and node1.val != node2.val):
return False
if (not node1 and not node2):
return True
return helper(node1.left, node2.right) and helper(node2.left, node1.right)
return helper(root.left, root.right)
```
---
### lowest common ancestor
```python
def lowest_common_ancestor(root, p, q):
stack = [root]
parent = {root: None}
while p not in parent or q not in parent:
node = stack.pop()
if node:
parent[node.left] = node
parent[node.right] = node
stack.append(node.left)
stack.append(node.right)
ancestors = set()
while p:
ancestors.add(p)
p = parent[p]
while q not in ancestors:
q = parent[q]
return q
```
---
### has path sum?
```python
def has_path_sum(root, target_sum) -> bool:
def transverse(node, sum_here=0):
if not node:
return sum_here == target_sum
sum_here += node.val
if not node.left:
return transverse(node.right, sum_here)
if not node.right:
return transverse(node.left, sum_here)
else:
return transverse(node.left, sum_here) or transverse(node.right, sum_here)
if not root:
return False
return transverse(root)
```
---
### build tree from inorder with preorder or postorder
* building with preorder:
```python
def build_tree(preorder, inorder) -> Optional[Node]:
def helper(left, right, index_map):
if left > right:
return None
root = Node(preorder.pop(0)) # this order change from postorder
index_here = index_map[root.val]
root.left = helper(left, index_here - 1, index_map) # this order change from postorder
root.right = helper(index_here + 1, right, index_map)
return root
index_map = {value: i for i, value in enumerate(inorder)}
return helper(0, len(inorder) - 1, index_map)
```
* build with postorder:
```python
def build_tree(left, right, index_map):
if left > right:
return None
root = Node(postorder.pop()) # this order change from preorder
index_here = index_map[root.val]
root.right = build_tree(index_here + 1, right, index_map) # this order change from preorder
root.left = build_tree(left, index_here - 1, index_map)
return root
def build_tree(inorder, postorder) -> Optional[Node]:
index_map = {val: i for i, value in enumerate(inorder)}
return fill_tree(0, len(inorder) - 1, index_map)
```
---
### return number of unival subtrees
* a unival subtree means all nodes of the subtree have the same value
```python
def count_unival(root) -> int:
global count = 0
def dfs(node):
if node is None:
return True
if dfs(node.left) and dfs(node.right):
if (node.left and node.left.val != node.val) or \
(node.right and node.right.val != node.val):
return False
self.count += 1
return True
return False
dfs(root)
return count
```
---
### successors and precessors
```python
def successor(root):
root = root.right
while root.left:
root = root.left
return root
def predecessor(root):
root = root.left
while root.right:
root = root.right
return root
```
----
### binary search trees
* **binary search tree** are binary trees where all nodes on the left are smaller than the root, which is smaller than all nodes on the right.
* if a bst is **balanced**, it guarantees `O(log(N))` for insert and search (as we keep the tree's height as `h = log(N)`).
* common types of balanced trees are **red-black** and **avl**.
---
### insert a node
* the main strategy is to find out a proper leaf position for the target and then insert the node as a leaf (therefore, insertion will begin as a search).
* the time complexity is `O(h)` where `h` is a tree height. that results in `O(log(N))` in the average case, and `O(N)` worst case.
```python
def bst_insert_iterative(root, val):
node = root
while node:
if val > node.val:
if not node.right:
node.right = Node(val)
break
else:
node = node.right
else:
if not node.left:
node.left = Node(val)
break
else:
node = node.left
return root
def bst_insert_recursive(root, val):
if root is None:
return Node(val)
if val > root.val:
root.right = self.bst_insert_recursive(root.right, val)
else:
root.left = self.bst_insert_recursive(root.left, val)
return root
```
---
### delete a node
* deletion is a more complicated operation, and there are several strategies.
* one of them is to replace the target node with a proper child:
- if the target node has no child (it's a leaf): simply remove the node
- if the target node has one child, use the child to replace the node
- if the target node has two child, replace the node with its in-order successor or predecessor node and delete the node
* similar to the recursion solution of the search operation, the time complexity is `O(h)` in the worst case.
* according to the depth of recursion, the space complexity is also `O(h)` in the worst case. we can also represent the complexity using the total number of nodes `N`.
* the time complexity and space complexity will be `O(log(N))` in the best case but `O(N)` in the worse case.
```python
def successor(root):
root = root.right
while root.left:
root = root.left
return root.val
def predecessor(root):
root = root.left
while root.right:
root = root.right
return root.val
def delete_node(root, key):
if root is None:
return root
if key > root.val:
root.right = delete_node(root.right, key)
elif key < root.val:
root.left = delete_node(root.left, key)
else:
if not (root.left or root.right):
root = None
elif root.right:
root.val = successor(root)
root.right = delete_node(root.right, root.val)
else:
root.val = predecessor(root)
root.left = delete_node(root.left, root.val)
return root
```
---
### search for a value
* for the recursive solution, in the worst case, the depth of the recursion is equal to the height of the tree. therefore, the time complexity would be `O(h)`. the space complexity is also `O(h)`.
* for an iterative solution, the time complexity is equal to the loop time which is also `O(h)`, while the space complexity is `O(1)`.
```python
def search_bst_recursive(root, val):
if root is None or root.val == val:
return root
if val > root.val:
return search_bst_recursive(root.right, val)
else:
return search_bst_recursive(root.left, val)
def search_bst_iterative(root, val):
while root:
if root.val == val:
break
if root.val < val:
root = root.right
else:
root = root.left
return root
```
---
### find successor of two nodes inorder
```python
def find_successor(node1, node2):
successor = None
while node1:
if node1.val <= node2.val:
node1 = node1.right
else:
successor = node1
node1 = node1.left
return successor
```
---
### convert sorted array to bst
* note that there is no unique solution.
```python
def convert_sorted_array_to_bst(nums):
def helper(left, right):
if left > right:
return None
p = (left + right) // 2
root = Node(nums[p])
root.left = helper(left, p - 1)
root.right = helper(p + 1, right)
return root
return helper(0, len(nums) - 1)
```
---
### lowest common ancestor for a bst
```python
def lowest_common_ancestor(root, p, q):
node, result = root, root
while node:
result = node
if node.val > p.val and node.val > q.val:
node = node.left
elif node.val < p.val and node.val < q.val:
node = node.right
else:
break
return result
```
---
### checking if bst is valid
```python
def is_valid_bst_iterative(root):
queue = deque((root, float(-inf), float(inf)))
while queue:
node, min_val, max_val = queue.popleft()
if node:
if min_val >= node.val or node.val >= max_val:
return False
if node.left:
queue.append((node.left, min_val, node.val))
if node.right:
queue.append((node.right, node.val, max_val))
return True
def is_valid_bst_recursive(root, min_val=float(-inf), max_val=float(inf)):
if root is None:
return True
return (min_val < root.val < max_val) and \
is_valid_bst_recursive(root.left, min_val, root.val) and \
is_valid_bst_recursive(root.right, root.val, max_val)
def is_valid_bst_inorder(root):
def inorder(node):
if node is None:
return True
inorder(node.left)
stack.append(node.val)
inorder(node.right)
stack = []
inorder(root)
for i in range(1, len(stack)):
if queue[i] <= queue[i - 1]:
return False
return True
```