leetcode 102 Binary Tree Level Order Traversal

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]
]

confused what

"{1,#,2,3}"

means?
> read more on how binary tree is serialized on OJ.

我的解决方案，非常传统的两个队列的解决方案：

/**
* 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>> result;
queue<TreeNode*> q;

if (root == NULL)
{
return result;
}
q.push(root);
vector<int> le_temp;

while(!q.empty())
{

le_temp.clear();
queue<TreeNode*> level;

int size = q.size();
for(int i = 0; i < size; ++i)
{
TreeNode* temp = q.front();
q.pop();
if(temp->left)
{
level.push(temp->left);
}
if(temp->right)
{
level.push(temp->right);

}
le_temp.push_back(temp->val);
}

while(!level.empty())
{

q.push(level.front());
level.pop();
}
result.push_back(le_temp);
}

return result;

}
};

一个栈似乎也行：

vector<vector<int>> levelOrder(TreeNode* root)
{

vector<vector<int> >  result;
if (!root) return result;
queue<TreeNode*> q;
q.push(root);
q.push(NULL);
vector<int> cur_vec;
while(!q.empty()) {
TreeNode* t = q.front();
q.pop();
if (t==NULL) {
result.push_back(cur_vec);
cur_vec.resize(0);
if (q.size() > 0) {
q.push(NULL);
}
} else {
cur_vec.push_back(t->val);
if (t->left) q.push(t->left);
if (t->right) q.push(t->right);
}
}
return result;

}

递归的解决方案：

class Solution {
public:
vector<vector<int>> result;
void buildVector(TreeNode* root, int depth)
{
if(root == NULL)return ;
if(result.size() == depth)
{
result.push_back(vector<int>());
}

result[depth].push_back(root->val);

buildVector(root->left,depth + 1);
buildVector(root->right, depth + 1);
}
vector<vector<int>> levelOrder(TreeNode* root)
{
buildVector(root,0);
return result;
}
};

逆序排列把return 改一下就好了： return
vector<vector<int> > (result.rbegin(), result.rend());

}