递归写法，非递归写法太麻烦，索性写了

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 * }
 */

public class Solution {
    /**
     * 
     * @param root TreeNode类 
     * @param o1 int整型 
     * @param o2 int整型 
     * @return int整型
     */
    public int lowestCommonAncestor (TreeNode root, int o1, int o2) {
        // write code here
        if (root == null) return -1;
        if (root.val == o1 || root.val == o2) return root.val;

        int left = lowestCommonAncestor(root.left, o1, o2);
        int right = lowestCommonAncestor(root.right, o1, o2);

        if (left != -1 && right != -1) return root.val;
        if (left != -1 && right == -1) return left;
        if (left == -1 && right != -1) return right;
        return -1;
    }
}