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=0;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=0;i<tmp->children.size();i++){
                  q.push(tmp->children[i]);
              }
            }
            v2.push_back(v1);
        }
        return v2;
    }
};