树————N叉树的层序遍历

思想：

使用队的思想，将每一层的节点放入队列中，依次弹出，同时将其children放入队列。

c++

 /*
 // Definition for a Node.
 class Node {
 public:
     int val;
     vector<Node*> children;

     Node() {}

     Node(int _val, vector<Node*> _children) {
         val = _val;
         children = _children;
     }
 };
 */
 class Solution {
 private:
     vector<vector<int> > res;
     vector<int> small_res;
     queue<Node*> que;
 public:
     vector<vector<int>> levelOrder(Node* root) {
         if(!root) return res;
         que.push(root);
         while(!que.empty()){
             small_res.erase(small_res.begin(),small_res.end());
             int s=que.size();
             for(int i=;i<s;i++){
                 Node *temp=que.front();
                 que.pop();
                 small_res.push_back(temp->val);
                 if(!temp->children.empty()){
                     for(Node* child: temp->children){
                         que.push(child);
                     }
                 }
             }
             res.push_back(small_res);
         }

         return res;
     }
 };

或者：（做了一些小改动）

 /*
 // Definition for a Node.
 class Node {
 public:
     int val;
     vector<Node*> children;

     Node() {}

     Node(int _val, vector<Node*> _children) {
         val = _val;
         children = _children;
     }
 };
 */
 class Solution {
 public:
     vector<vector<int>> levelOrder(Node* root) {
         vector<int>v1;
         vector<vector<int>>v2;
         if(root==NULL) return v2;
         queue<Node*>q;
         q.push(root);
         while(!q.empty()){
             int cnt=q.size();
             v1.clear();
             while(cnt--){
               Node *tmp=q.front();
               v1.push_back(tmp->val);
               q.pop();
               for(int i=;i<tmp->children.size();i++){
                   q.push(tmp->children[i]);
               }
             }
             v2.push_back(v1);
         }
         return v2;
     }
 };