LeetCode 102. Binary Tree Level Order Traversal 二叉树的层次遍历 C++

Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).

For example:
Given binary tree [3,9,20,null,null,15,7]


   / \

    /  \
      

return its level order traversal as:

[
  [],
  [,],
  [,]
]

二叉树的层次遍历使用队列来实现，很大程度上类似于求树的最大深度。

第一种解法：开始时当前结点数curLevelCount=1,nextLevelCount=0,根据curLevelCount来输入每一层的结点值，当其左右子树存在时，将左右子树存入队列，并nextLevelCount++，每弹出一个当前层的结点时，curLevelCount--，当其为0时，再创建一个新的vector。

C++：

 vector<vector<int>> levelOrder(TreeNode* root) {
         vector<vector<int>> res={};
         queue<TreeNode*> q;
         if(!root)
             return res;
         q.push(root);
         int curLevelCount=,nextLevelCount=;
         vector<int> temp={};
         while(!q.empty()){
             TreeNode* cur=q.front();
             temp.push_back(cur->val);
             q.pop();
             curLevelCount--;
             if(cur->left){
                 q.push(cur->left);
                 nextLevelCount++;
             }
             if(cur->right){
                 q.push(cur->right);
                 nextLevelCount++;
             }
             if(curLevelCount==){
                 res.push_back(temp);
                 temp={};
                 curLevelCount=nextLevelCount;
                 nextLevelCount=;
             }
         }
         return res;
     }

第二种方法不用参数计算每一层结点个数，利用for循环，每一层时利用队列的大小来计算每一层的结点数。两层循环嵌套，效率比较低。

 vector<vector<int>> levelOrder(TreeNode* root) {
         vector<vector<int>> res={};
         queue<TreeNode*> q;
         if(!root)
             return res;
         q.push(root);
         while(!q.empty()){
             vector<int> temp={};
             for(int i=q.size();i>;i--){
                 TreeNode* cur=q.front();
                 temp.push_back(cur->val);
                 q.pop();
                 if(cur->left)
                     q.push(cur->left);
                 if(cur->right)
                     q.push(cur->right);
             }
             res.push_back(temp);
         }
         return res;
     }