101.对称二叉树

https://leetcode-cn.com/problems/symmetric-tree/submissions/

用bfs层次遍历，队列实现

class Solution {
public:
    bool isSymmetric(TreeNode* root) {
        if(!root) return true;
        const int MIN=-0x3f3f3f3f;
        queue<TreeNode*> q;
        q.push(root);
        while(!q.empty())
        {
            int length = q.size();
            vector<int> vis(length);
            for(int i = 0; i<length; ++i)
            {
                root = q.front();
                q.pop();
                vis[i] = (root!=NULL ? root->val : MIN);
                if(root) { q.push(root->left); q.push(root->right); }
            }
            for(int i = 0; i< length/2; ++i)
            {
                if(vis[i] != vis[length-1-i]) 
                return false;
            }
        }
        return true;
    }
};