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/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
/*
    bool isSymmetric(TreeNode* root) {
        return helper(root,root);
    }
*/

    bool isSymmetric(TreeNode* root) {
        if(!root)return true;
        //return func(root->left, root->right);
        queue<TreeNode*>q;
        q.push(root->left);
        q.push(root->right);
        while(!q.empty()){
            auto l=q.front();q.pop();
            auto r=q.front();q.pop();
            if(!l && !r)
                continue;
            else if( !l || !r)
                return false;
            else{
                if(l->val != r->val)
                    return false;
                q.push(l->left);
                q.push(r->right);
                q.push(l->right);
                q.push(r->left);
            }
        }
        return true;
    }
    
    bool helper(TreeNode* l,TreeNode* r){
        if(!l&&!r)return true;
        if(l==nullptr||r==nullptr)return false;
        return (l->val==r->val)&&helper(l->left,r->right)&&helper(l->right,r->left);
    }
};