python 最近公共祖先在二叉树中和在二叉搜索树中 LeetCode No.236 No.235

本文链接：https://blog.youkuaiyun.com/GhostintheCode/article/details/98856339

本文详细介绍了如何使用递归方法找到二叉树中两个节点的最低公共祖先，同时探讨了二叉搜索树的特殊情况，并提供了非递归实现方式。

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思路：通过递归的思路写这道题就非常容易，想象一下某一个状态，p,q 分别在root的两边，那代表什么？是不是代表，p，q是在root点分开的。那我们就找到了这个点。那再想象一下，如果p,q都在同一边，那我们就去找边找，比如，p，q都在左子树，那么我们就把根节点左子树传下去，右边同理。

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def lowestCommonAncestor(self, root, p, q):
        """
        :type root: TreeNode
        :type p: TreeNode
        :type q: TreeNode
        :rtype: TreeNode
        """
        if root is None or root == p or root == q:
            return root
        left = self.lowestCommonAncestor(root.left, p , q) 
        right = self.lowestCommonAncestor(root.right, p, q)
       # if left is None:
        #     return right
        # elif right is None:
        #     return left
        # else:
        #     return root
        #下面一行写法，展开在上面?
        return right if left is None else left if right is None else root

同样的思路也可以写235题，只不过不是二叉树，是二叉搜索树。可以根据二叉搜索树的性质。

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def lowestCommonAncestor(self, root, p, q):
        """
        :type root: TreeNode
        :type p: TreeNode
        :type q: TreeNode
        :rtype: TreeNode
        """
        #  if p.val< root.val >q.val:
        #     return self.lowestCommonAncestor(root.left, p, q)
        # if p.val> root.val <q.val:
        #     return self.lowestCommonAncestor(root.right, p, q)
        #return root
        #一行写法，拓展如上
        return self.lowestCommonAncestor(root.left, p, q) if p.val< root.val >q.val else  self.lowestCommonAncestor(root.right, p, q) if p.val> root.val <q.val else root

#非递归写法
# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def lowestCommonAncestor(self, root, p, q):
        """
        :type root: TreeNode
        :type p: TreeNode
        :type q: TreeNode
        :rtype: TreeNode
        """
        while root:
            if p.val <root.val >q.val:
                root = root.left
            elif p.val> root.val < p.val:
                root = root.right
            else:
                return root