leetCode——二叉树的层次遍历

题目：https://leetcode-cn.com/explore/learn/card/data-structure-binary-tree/2/traverse-a-tree/9/

class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) { 
        vector<vector<int>> ans;
        if(!root) return ans;
        queue<TreeNode*> q;
        q.push(root);
        TreeNode* tr;
        while(!q.empty()){
            int n = q.size();
            vector<int> v;
            for(int i = 0;i<n;i++){
                tr = q.front();
                v.push_back(tr->val);
                q.pop();
                if(tr->left) q.push(tr->left);
                if(tr->right) q.push(tr->right);
            }
            ans.push_back(v);
        }
        return ans;
    }
};