Leetcode---Symmetric Tree

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

For example, this binary tree is symmetric:

    1
   / \
  2   2
 / \ / \
3  4 4  3

But the following is not:

    1
   / \
  2   2
   \   \
   3    3

判断一棵树是不是镜像。关于树，直接的想法就是递归，但是这个题目从单个树来看是无法递归的，但是可以用两个树来递归。

bool f(TreeNode * l, TreeNode * r){
    if(l==NULL && r==NULL)
        return true;
    if(l==NULL || r==NULL)
        return false;
    return l->val==r->val && f(l->left,r->right)&&
        f(l->right,r->left);
}
bool isSymmetric(TreeNode *root) {
    if(root==NULL)
        return true;
    return f(root->left,root->right);
}