【LEETCODE】236-Lowest Common Ancestor of a Binary Tree

Given a binary tree, find the lowest common ancestor (LCA) of two given nodes in the tree.

According to the definition of LCA on Wikipedia: “The lowest common ancestor is defined between two nodes v and w as the lowest node in T that has both v and w as descendants (where we allow a node to be a descendant of itself).”

        _______3______

       /              \

    ___5__          ___1__

   /      \        /      \

   6      _2       0       8

         /  \

         7   4

For example, the lowest common ancestor (LCA) of nodes 5 and 1 is 3. Another example is LCA of nodes 5 and 4 is 5, since a node can be a descendant of itself according to the LCA definition.

参考：

https://github.com/kamyu104/LeetCode/blob/master/Python/lowest-common-ancestor-of-a-binary-tree.py

# 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 in (None,p,q):          #找p，q的node
            return root
        
        left=self.lowestCommonAncestor(root.left,p,q)            #左右遍历直到找到p或者q或者二者
        right=self.lowestCommonAncestor(root.right,p,q)
        
        if left and right:              #当p，q位于root的左右两边时，返回root
            return root
        else:                           #此外情况，说明p，q位于root的某同一侧子树，则先遇到p，q的哪个就返回哪个
            return left or right