/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        if (root == null) return null;
        // 如果p,q为根节点，则公共祖先为根节点
        if (root.val == p.val || root.val == q.val) return root;
        // 如果p,q在左子树，则公共祖先在左子树查找
        if (find(root.left, p) && find(root.left, q)) {
            return lowestCommonAncestor(root.left, p, q);
        }
        // 如果p,q在右子树，则公共祖先在右子树查找
        if (find(root.right, p) && find(root.right, q)) {
            return lowestCommonAncestor(root.right, p, q);
        }
        // 如果p,q分属两侧，则公共祖先为根节点
        return root;
    }
    
    private boolean find(TreeNode root, TreeNode c) {
        if (root == null) return false;
        if (root.val == c.val) {
            return true;
        }
        
        return find(root.left, c) || find(root.right, c);
    }
}