LeetCode - Binary Tree Level Order Traversal II

二叉树的层序遍历

思路一：利用队列，将每一层节点放入队列，各层节点之间加入NULL隔开。

 1 /**
 2  * Definition for binary tree
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 public:
12     vector<vector<int> > levelOrderBottom(TreeNode *root) {
13         vector<vector<int> > levelOrderVec;
14         if (root == NULL)
15             return levelOrderVec;
16         queue<TreeNode *> treeQueue;
17         treeQueue.push(root);
18         treeQueue.push(NULL);
19         vector<int> level;
20         while (!treeQueue.empty())
21         {
22             TreeNode *tmp = treeQueue.front();
23             treeQueue.pop();
24             if (tmp != NULL)
25             {
26                 level.push_back(tmp->val);
27                 if (tmp->left)
28                     treeQueue.push(tmp->left);
29                 if (tmp->right)
30                     treeQueue.push(tmp->right);
31             }
32             else
33             {
34                 if (level.size() != 0)
35                 {
36                     levelOrderVec.push_back(level);
37                     treeQueue.push(NULL);
38                 }
39                 level.clear();
40             }
41         }
42         
43         reverse(levelOrderVec.begin(), levelOrderVec.end());
44         
45         return levelOrderVec;
46     }
47 };

思路二：深度优先搜索

 1 /**
 2  * Definition for binary tree
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 public:
12     vector<vector<int> > levelOrderBottom(TreeNode *root) {
13         DFS(root, 0);
14         return vector<vector<int> > (res.rbegin(), res.rend());
15     }
16     
17     void DFS(TreeNode *root, int level)
18     {
19         if (root == NULL) return;
20         // 当前level不存在
21         // 新建一个
22         if (level == res.size())
23         {
24             res.push_back(vector<int>());
25         }
26         
27         res[level].push_back(root->val);
28         DFS(root->left, level+1);
29         DFS(root->right, level+1);
30     }
31     
32 private:
33     vector<vector<int> > res;
34 };

 

posted @ 2015-04-04 18:59  bournet  阅读(98)  评论(0)  编辑  收藏  举报