LeetCode之“树”：Binary Tree Level Order Traversal && Binary Tree Level Order Traversal II

Binary Tree Level Order Traversal

　　题目链接

　　题目要求：

　　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,#,#,15,7},

    3
   / \
  9  20
    /  \
   15   7

　　return its level order traversal as:

[
  [3],
  [9,20],
  [15,7]
]

　　这道题利用宽度优先搜索就可以了，具体程序（8ms）如下：

 /**
  * 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<vector<int>> levelOrder(TreeNode* root) {
         vector<vector<int>> retVec;
         if(!root)
             return retVec;

         retVec.push_back(vector<int>{root->val});
         queue<TreeNode *> que;
         que.push(root);
         while(true)
         {
             vector<int> vec;
             queue<TreeNode *> q;
             while(!que.empty())
             {
                 TreeNode *tree = que.front();
                 que.pop();
                 if(tree->left)
                 {
                     vec.push_back((tree->left)->val);
                     q.push(tree->left);
                 }
                 if(tree->right)
                 {
                     vec.push_back((tree->right)->val);
                     q.push(tree->right);
                 }
             }

             if(!q.empty())
             {
                 que = q;
                 retVec.push_back(vec);
             }
             else
                 break;
         }

         return retVec;
     }
 };

Binary Tree Level Order Traversal II

　　题目链接

　　题目要求：

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

　　For example:
　　Given binary tree {3,9,20,#,#,15,7},

    3
   / \
  9  20
    /  \
   15   7

　　return its bottom-up level order traversal as:

[
  [15,7],
  [9,20],
  [3]
]

　　这道题基本更上边的题目一样，我们只需要将上题的结果retVec最后再反转一下就可以了，即添加如下一行即可：

 reverse(retVec.begin(), retVec.end());

　　这样的程序只要8ms，但下边基本一样的程序却要64ms：

 /**
  * 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<vector<int>> levelOrderBottom(TreeNode* root) {
         vector<vector<int>> retVec;
         if(!root)
             return retVec;

         retVec.insert(retVec.begin(), vector<int>{root->val});
         queue<TreeNode *> que;
         que.push(root);
         while(true)
         {
             vector<int> vec;
             queue<TreeNode *> q;
             while(!que.empty())
             {
                 TreeNode *tree = que.front();
                 que.pop();
                 if(tree->left)
                 {
                     vec.push_back((tree->left)->val);
                     q.push(tree->left);
                 }
                 if(tree->right)
                 {
                     vec.push_back((tree->right)->val);
                     q.push(tree->right);
                 }
             }

             if(!q.empty())
             {
                 que = q;
                 retVec.insert(retVec.begin(), vec);
             }
             else
                 break;
         }

         return retVec;
     }
 };