Binary Tree Zigzag Level Order Traversal

Binary Tree Zigzag Level Order Traversal

Given a binary tree, return the zigzag level order traversal of its nodes' values. (ie, from left to right, then right to left for the next level and alternate between).

For example:

Given binary tree

{3,9,20,#,#,15,7}

,

3
/ \
9  20
/  \
15   7

return its zigzag level order traversal as:

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

BFS,宽度查找法，把同一级别的宽度记下来，访问的同时把下一层弄到队列里面去。常用遍历方法!

class Solution {
public:
vector<vector<int>> zigzagLevelOrder(TreeNode* root)
{
vector<vector<int>> levelOrderStore;
if (root==NULL)
return levelOrderStore;
queue<TreeNode *>q;
stack<TreeNode *>rightout;
q.push(root);
rightout.push(root);
int count=1;
int level=0;
vector<int> tmp(0);
int flag=0;//begin  right->left->right
while (!q.empty())//q.root
{
tmp.clear();
level=0;
for (int i=0;i<count;i++)//level
{

root=q.front(); //every root.
q.pop();

if (flag%2==0)
{
tmp.push_back(root->val);
}
else
{
tmp.push_back(rightout.top()->val); //every root.
rightout.pop();//
}

if (root->left!=NULL)
{
q.push(root->left);
if (flag%2==0)
{
rightout.push(root->left); //
}
++level;
}
if (root->right!=NULL)
{
q.push(root->right);
if (flag%2==0)
rightout.push(root->right);
++level;
}
}
count=level;
levelOrderStore.push_back(tmp);
flag++;
}

return levelOrderStore;
}

};