101. Symmetric Tree

问题描述：

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

For example, this binary tree [1,2,2,3,4,4,3] is symmetric:

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

But the following [1,2,2,null,3,null,3] is not:

    1   / \  2   2   \   \   3    3

解题思路：

对称二叉树，同等价二叉树思路一样，直接给出左左子树与右右子树进行比较是否等价。

class Solution {
public:
    bool isSymmetric(TreeNode* root) {
        if(root==NULL) return true;
        //else if(!root->left&&!root->right) return true;
        else return sureSymmetric(root->left,root->right);
    }
    bool sureSymmetric(TreeNode* left,TreeNode* right)
    {
        if(left==NULL&&right==NULL)
            return true;
        else if((left!=NULL&&right==NULL)||(left==NULL&&right!=NULL)||left->val!=right->val)
            return false;
        else return sureSymmetric(left->left,right->right)&&sureSymmetric(left->right,right->left);
    }
};