思路：使用map存储每个节点的父节点，则两个节点的最近公共祖先，即二者的最近父节点

1、中序遍历二叉树（当前节点的下一个节点）
2、记录每个节点的父节点
3、列出p的族谱、q的族谱
4、寻找二者最近的祖先

class Solution {
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode n1, TreeNode n2) {
        if (n1.val == root.val || n2.val == root.val) {
            return root;
        }

        // 1.寻找所所有节点的父节点
        Map<TreeNode, TreeNode> map = new HashMap<>();
        findParent(map, root);

        // 2.n1的家谱（包括自己）
        List<TreeNode> n1ParentList = getPeople(map, n1);

        // 3.n2的家谱
        List<TreeNode> n2ParentList = getPeople(map, n2);

        // 4.n1和n2的公共长辈的第一个长辈
        n1ParentList.retainAll(n2ParentList);
        return n1ParentList.get(0);
    }

    private void findParent(Map<TreeNode, TreeNode> map, TreeNode root) {
        map.put(root, null);
        TreeNode temp = root;

        // 1.最左子节点
        while (temp.left != null) {
            map.put(temp.left, temp);
            temp = temp.left;
        }

        // 2.下一个节点
        TreeNode curNode = temp;
        while (true) {
            TreeNode nextNode = next(map, curNode);
            if (nextNode == null) {
                return;
            }
            curNode = nextNode;
        }
    }

    private TreeNode next(Map<TreeNode, TreeNode> map, TreeNode curNode) {
        if (curNode.right != null) {
            TreeNode next = curNode.right;
            map.put(next, curNode);
            while (next.left != null) {
                map.put(next.left, next);
                next = next.left;
            }
            return next;
        } else {
            TreeNode parent = map.get(curNode);
            while (true) {
                if (parent == null) {
                    return null;
                } else if (parent.left != null && Objects.equals(parent.left.val, curNode.val)) {
                    return parent;
                } else {
                    curNode = parent;
                    parent = map.get(curNode);
                }
            }
        }
    }

    private List<TreeNode> getPeople(Map<TreeNode, TreeNode> map, TreeNode node) {
        List<TreeNode> list = new ArrayList<>();
        while (node != null) {
            list.add(node);
            TreeNode parent = map.get(node);
            node = parent;
        }
        return list;
    }
}