669. Trim a Binary Search Tree - Explanation

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def trimBST(self, root: Optional[TreeNode], low: int, high: int) -> Optional[TreeNode]:
        if not root:
            return None

        if root.val > high:
            return self.trimBST(root.left, low, high)
        if root.val < low:
            return self.trimBST(root.right, low, high)

        root.left = self.trimBST(root.left, low, high)
        root.right = self.trimBST(root.right, low, high)
        return root

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def trimBST(self, root, low, high):
        while root and (root.val < low or root.val > high):
            if root.val < low:
                root = root.right
            else:
                root = root.left

        stack = [root]
        while stack:
            node = stack.pop()
            if not node:
                continue
            left_out = node.left and node.left.val < low
            right_out = node.right and node.right.val > high
            if left_out:
                node.left = node.left.right
            if right_out:
                node.right = node.right.left
            if left_out or right_out:
                stack.append(node)
            else:
                if node.left:
                    stack.append(node.left)
                if node.right:
                    stack.append(node.right)
        return root

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def trimBST(self, root: Optional[TreeNode], low: int, high: int) -> Optional[TreeNode]:
        while root and (root.val < low or root.val > high):
            root = root.right if root.val < low else root.left

        tmpRoot = root
        while root:
            while root.left and root.left.val < low:
                root.left = root.left.right
            root = root.left

        root = tmpRoot
        while root:
            while root.right and root.right.val > high:
                root.right = root.right.left
            root = root.right

        return tmpRoot