Binary Tree Level Order Traversal

问题：从上到下打印二叉树的每一行
分析：先搜出二叉树的高度，然后遍历高度，每次搜索一个高度

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class Solution { public:     int dfs(TreeNode *root)     {         if(root==NULL) return 0;         if(root->left==NULL && root->right==NULL) return 1;         return max(dfs(root->left),dfs(root->right))+1;     }     void Ddfs(TreeNode *root,int t,vector<int> &vec,int step)     {         if(step==t) vec.push_back(root->val);         if(root->left)  Ddfs(root->left,t,vec,step+1);         if(root->right) Ddfs(root->right,t,vec,step+1);     }     vector<vector<int> > levelOrder(TreeNode *root) {         int height=dfs(root);         vector<vector<int> > vec1;         for(int i=1;i<=height;i++)         {             vector<int> vec2;             Ddfs(root,i,vec2,1);             vec1.push_back(vec2);         }         return vec1;     } };