Solution.js

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @param {number} n
 * @param {number} x
 * @return {boolean}
 */
var btreeGameWinningMove = function (root, n, x) {
    const dfs = root => {
        if (!root || root.val === x) {
            return root;
        }
        return dfs(root.left) || dfs(root.right);
    };

    const count = root => {
        if (!root) {
            return 0;
        }
        return 1 + count(root.left) + count(root.right);
    };

    const node = dfs(root);
    const l = count(node.left);
    const r = count(node.right);
    return Math.max(l, r, n - l - r - 1) > n / 2;
};