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

判断一棵二叉树是否是镜像的

C++（15ms）：

/**
 * 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) {
        TreeNode* left ;
        TreeNode* 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 ;
    }
};