450. Delete Node in a BST - LeetCode Solution

# 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 deleteNode(self, root: TreeNode, key: int) -> TreeNode:
        if not root:
            return root
        if key < root.val:
            root.left = self.deleteNode(root.left, key)
        elif key > root.val:
            root.right = self.deleteNode(root.right, key)
        else:
            if not root.left:
                return root.right
            if not root.right:
                return root.left


            pre = root
            nex = root.right
            while nex.left:
                pre = nex
                nex = nex.left

            root.val = nex.val 
            if pre != root:
                pre.left = nex.right
            else:
                pre.right = nex.right 

        return root