对称二叉树 ​

var isSymmetric = function(root) {
    return cmp(root.left,root.right)
};

var cmp = function(left,right) {
    if(left===null&&right!=null) return false
    if(right===null&&left!=null) return false
    if(right===null&&left===null) return true
    if(left.val!=right.val)   return false
    //left、right都存在并且值相等
    let outside = cmp(left.left, right.right);  // 比较外侧
    let inside = cmp(left.right, right.left);   // 比较内侧
    return outside && inside;
};