Leetcode-236:二叉树的最近公共祖先

题目描述：
给定一个二叉树, 找到该树中两个指定节点的最近公共祖先。

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