[LeetCode107]Binary Tree Level Order Traversal II 二叉树层次遍历

题目：

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},

return its bottom-up level order traversal as:

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

分类：Tree BFS

代码：

递归：

 1 class Solution {
 2 protected:
 3     vector<vector<int>> ans;
 4     void dfs(TreeNode *root, int height){
 5         if (root == NULL) 
 6             return;
 7         while (ans.size() <= height)
 8             ans.push_back(vector<int>());
 9         ans[height].push_back(root->val);
10         dfs(root->left, height + 1);
11         dfs(root->right, height + 1);
12     }
13 
14 public:
15     vector<vector<int>> levelOrderBottom(TreeNode* root) {
16        dfs(root, 0);
17        reverse(ans.begin(), ans.end());
18         return ans;    
19     }
20 };

非递归：

 1 /**
 2  * Definition for a binary tree node.
 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>> resultVec;
14         if(!root)
15             return resultVec;
16         int parentSize = 1, childSize = 0;
17         int level = 0;
18         TreeNode *temp;
19         queue<TreeNode*> nodes;
20         nodes.push(root);
21         resultVec.push_back(vector<int>());
22         while(!nodes.empty())
23         {
24             temp  = nodes.front();
25             resultVec[level].push_back(temp->val);
26             nodes.pop();
27             if(temp->left)
28             {
29                 nodes.push(temp->left);
30                 childSize++;
31             }
32             if(temp->right)
33             {
34                 nodes.push(temp->right);
35                 childSize++;
36             }
37             parentSize--;
38             if(parentSize == 0)
39             {
40                 parentSize = childSize;
41                 childSize = 0;
42                 level++;
43                 resultVec.push_back(vector<int>());
44             }
45         }
46         resultVec.erase(resultVec.end()-1);
47         reverse(resultVec.begin(),resultVec.end());
48         return resultVec;
49     }
50 };

[LeetCode107]Binary Tree Level Order Traversal II 二叉树层次遍历

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