解题思路：

后序遍历

注意：

p和q其中一个就是它们的公共祖先的情况也考虑到了，假设q是公共祖先，遇到q就直接返回，相当于是下面一边为空，一边不为空的情况，返回不为空就一边即可

class Solution {
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        return recur(root, p, q);
    }

    public TreeNode recur(TreeNode root, TreeNode p, TreeNode q) {
        if (root == null) return null;
        if (root == p || root == q) return root;

        TreeNode left = recur(root.left, p, q);
        TreeNode right = recur(root.right, p, q);
        if (left != null && right != null) return root;
        if (left == null && right != null) return right;
        if (left != null && right == null) return left;
        else return null;
    }
}