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
 * @return {boolean}
 */
var isSymmetric = function (root) {
    function dfs(root1, root2) {
        if (!root1 && !root2) return true;
        if (!root1 || !root2 || root1.val != root2.val) return false;
        return dfs(root1.left, root2.right) && dfs(root1.right, root2.left);
    }
    return dfs(root, root);
};