leetcode 二叉树层次遍历输出Binary Tree Level Order Traversal

逆序输出：

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]

]

递归进行层次遍历，每层相当于一个vector<int>，由于与层数有关故引入层数参数：

class Solution {
private:
vector<vector<int>>res;
public:
void levelOrder(struct TreeNode* root,int level){
if(root==NULL)return ;
if(level==res.size()){
vector<int> v;
res.push_back(v);
}
res[level].push_back(root->val);
levelOrder(root->left,level+1);
levelOrder(root->right,level+1);
}//确定是level层而不是level-1层
vector<vector<int>> levelOrderBottom(struct TreeNode* root) {
levelOrder(root,0);
return vector<vector<int>>(res.rbegin(),res.rend());
}
};

顺序输出：

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]

]

class Solution {
private:
vector<vector<int>>res;
public:
void levelOrder(struct TreeNode* root,int level){
if(root==NULL)return ;
if(level==res.size()){
vector<int> v;
res.push_back(v);
}
res[level].push_back(root->val);
levelOrder(root->left,level+1);
levelOrder(root->right,level+1);
}
vector<vector<int>> levelOrder(TreeNode* root) {
levelOrder(root,0);
return res;
}
};