5月20更新：

使用借助队列实现bfs，定义len记录队列的尺寸直接进行遍历层序

/** * Definition for a binary tree node. * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; *//**采用一个队列，使用一个变量记录上一层的元素个数。**/class Solution {public:    vector
     
      
       > levelOrder(TreeNode* root) {        vector
       
        
         > res;        if(root==NULL) return res;        queue
         
           q;        q.push(root);        while(!q.empty()){            vector
          
            level; int len=q.size(); for(int i=0;i
           
            val); if(p->left!=NULL) q.push(p->left); if(p->right!=NULL) q.push(p->right); } res.push_back(level); } return res; }};
           
          
         
        
       
      
     

 

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更新之前：使用广度优先搜索和获得队列大小：

/** * Definition for a binary tree node. * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    vector
     
      
       > levelOrder(TreeNode* root) {        if(root==NULL) return {};        queue
       
         q;        TreeNode* front;        q.push(root);        vector
        
         
          > res;                while(!q.empty()){            vector
          
            onelevel; for(int i=q.size();i>0;i--){ front=q.front(); q.pop(); if(front->left) q.push(front->left); if(front->right) q.push(front->right); onelevel.push_back(front->val); } res.push_back(onelevel); } return res; }};
          
         
        
       
      
     

使用两个队列：

/** * Definition for a binary tree node. * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    vector
     
      
       > levelOrder(TreeNode* root) {        vector
       
        
         > res;        queue
         
           q1;        queue
          
            q2; if(root==NULL) return {}; q1.push(root); while(!q1.empty()){ vector
           
             level; while(!q1.empty()){ TreeNode* cur=q1.front(); q1.pop(); level.push_back(cur->val); if(cur->left) q2.push(cur->left); if(cur->right) q2.push(cur->right); } while(!q2.empty()){ q1.push(q2.front()); q2.pop(); } res.push_back(level); } return res; }};