101. Symmetric Tree

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

/**
 * 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) {
        if(!root)
        return true;
        else 
        return is(root->left, root->right);
        
    }
        
    bool is(TreeNode* left, TreeNode* right){
        
        if(left == NULL || right == NULL)
        return (left == right) ;
        
        if(left->val != right->val)
        return false;
        
        return is(left->left, right->right) && is(left->right, right->left);
        
        
    }

};

class Solution {
public:
    bool isSymmetric(TreeNode* root) {
        TreeNode *left, *right;
        if(!root)
        return true;
        queue<TreeNode*>q1, q2;
        q1.push(root->left);
        q2.push(root->right);
        while(!q1.empty() && !q2.empty()){
            left = q1.front();
            q1.pop();
            right = q2.front();
            q2.pop();
            if(left == NULL && right == NULL)
            continue;
            if(left == NULL || right == NULL)
            return false;
            if(left->val != right->val)
            return false;
            q1.push(left->left);
            q1.push(left->right);
            q2.push(right->right);
            q2.push(right->left);
        }
        return true;
    }
    
};