/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
 //用f(x)表示x节点的子树是否包含p或者q
 //则满足公共祖先节点有两种情况组成
 //1.f(x.left) && f(x.right)
 //2.f(x.left) || f(x.right) && x==另一个
class Solution {
    TreeNode ans = new TreeNode();
    public boolean dfs(TreeNode root, TreeNode p, TreeNode q){
        if(root == null) return false;
        boolean lson = dfs(root.left, p, q);
        boolean rson = dfs(root.right, p, q);
        //情况1
        if(lson && rson){
            ans = root;
        }
        //情况2
        else if((lson || rson) && (root.val == q.val || root.val == p.val)){
            ans = root;
        }
        //因为所有结点的val都不相同，所以直接返回就行，不用考虑复杂情况
        return rson || lson || (root.val == p.val || root.val == q.val);
    }
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        dfs(root, p, q);
        return ans;
    }
}