LCA(最近公共祖先)（leetcode 236 python C++）

LCA（Lowest Common Ancestors），即最近公共祖先，是指在有根树中，找出某两个结点u和v最近的公共祖先。

# 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
        """
        #LCA 问题
        #利用先序遍历，寻找这两个节点 
        if not root or root == p or root == q: #判断这三种情况，找到节点或者没有，若存在这个节点，则返回这个节点，若没有返回None
            return root
        left = self.lowestCommonAncestor(root.left, p, q)
        right = self.lowestCommonAncestor(root.right, p, q)
        
        if left and right: #在root两边分别找到了节点
            return root
        if not left: #找到了right节点，right节点为父节点
            return right
        if not right:
            return left

　C++代码

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
        if (!root || root == p || root == q)
            return root;
        TreeNode* left = lowestCommonAncestor(root->left,p,q);
        TreeNode* right = lowestCommonAncestor(root->right,p,q);
        if (left && right)
            return root;
        if (!left)
            return right;
        if (!right)
            return left;

    }
};