解题思路:1.调整p,q使得p<=q

2.递归问题定义,root为树中找pq的父节点;如果root.val小于p证明root.val的值太小,遍历右子树。如果root.val的值大于q证明值太大遍历左子树

3.递归边界,如果当前root.val大于等于且root.val小于等于q,就是答案

import java.util.*;
public class Solution {
    public int lowestCommonAncestor (TreeNode root, int p, int q) {
        if(p>q){
            int t = p;
            p = q;
            q = t;
        }
        if(p <= root.val && root.val <= q)return root.val;
        if(q<root.val)return lowestCommonAncestor(root.left,p,q);
        return lowestCommonAncestor(root.right,p,q);
    }
}