leetcode hot 02

解题思路：找祖先从底向上递归后序遍历查找，遇到p，q或者空节点就直接返回对应值，当某个节点的左子树、右子树都返回了值，那么就说明该节点就是最近祖先节点，然后把该节点的值继续往上传，直到根节点返回结果。

/**
 * 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 == p || root==q || root==null) return root;
        TreeNode left = lowestCommonAncestor(root.left,p,q);
        TreeNode right = lowestCommonAncestor(root.right,p,q);
        if(left==null && right==null) return null;
        if(left==null && right!= null) return right;
        if(left!=null && right==null) return left;
        else return root;
    }
}