101. Symmetric Tree

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
static int wing=[]()
{
    std::ios::sync_with_stdio(false);
    cin.tie(NULL);
    return 0;
}();

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

用个镜像函数来递归判定，大树化小树，经典递归