1. let visited;let parent;
    2. var lowestCommonAncestor = function(root, p, q) {
    3.     visited = new Set();
    4.     parent = new Map();
    5.     dfs(root);
    6.     while (p != null) {
    7.         visited.add(p.val);
    8.         p = parent.get(p.val);
    9.     }
    10.     while (q != null) {
    11.         if (visited.has(q.val)) {
    12.             return q;
    13.         }
    14.         q = parent.get(q.val);
    15.     }
    16.     return null;
    17. };
    18. function dfs(root) {
    19.     if (root.left != null) {
    20.         parent.set(root.left.val, root);
    21.         dfs(root.left);
    22.     }
    23.     if (root.right != null) {
    24.         parent.set(root.right.val, root);
    25.         dfs(root.right);
    26.     }
    27. }