LeetCode 刷题笔记 之 Lowest Common Ancestor of a Binary Tree

题目如下：

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).”

Given the following binary search tree:  root = [3,5,1,6,2,0,8,null,null,7,4]

        _______3______
       /              \
    ___5__          ___1__
   /      \        /      \
   6      _2       0       8
         /  \
         7   4

Example 1:

Input: root, p = 5, q = 1
Output: 3
Explanation: The LCA of of nodes 5 and 1 is 3.

Example 2:

Input: root, p = 5, q = 4
Output: 5
Explanation: The LCA of nodes 5 and 4 is 5, since a node can be a descendant of itself
             according to the LCA definition.

解答如下：

使用递归的方法，找到两个节点所在分支。

代码：

class Solution {
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        if (root==null || p == root || q == root) return root;
       TreeNode left = lowestCommonAncestor(root.left, p, q);
       TreeNode right = lowestCommonAncestor(root.right, p , q);
       if (left!=null&&right!=null) return root;
       return left==null?right:left;
    }