Check a binary tree for symmetry

is-symmetric-tree.js

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {boolean}
 */
const isSymmetric = (root) => {
    if (root == null) {
        return true;
    }
    
    return isMirrored(root.left, root.right)
};

const isMirrored = (left, right) => {
    if (left == null && right == null) {
        return true;
    }
    
    if (left == null || right == null) {
        return false;
    }
    
    if (left.val != right.val) {
        return false;
    }
    
    return isMirrored(left.left, right.right) && isMirrored(left.right, right.left);
}