解题思路：

dfs

节点p,q异侧时，节点root为它们的公共祖先。

class Solution {
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        if (root == null || p == root || q == root) 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) return right;
        if (right == null) return left;

        return root;

    }
}