发现是二叉搜索树，递归写起来更简单

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param root TreeNode类 
     * @param p int整型 
     * @param q int整型 
     * @return int整型
     */
    public int lowestCommonAncestor (TreeNode root, int p, int q) {
        // write code here
        if (root == null) return -1;
        int s = root.val;
        if ((s <= p && s >= q) || (s >= p && s <= q)) return s;
        if (s < p && s < q) return lowestCommonAncestor(root.right, p, q);
        else return lowestCommonAncestor(root.left, p, q);
    }
}