之字形层次遍历二叉树：层次遍历二叉树，并且奇数行为从左到右（根节点为第一层），偶数行为从右到左。
先写一个容易实现的，参照《编程之美》3.10分层遍历二叉树的做法，先编写一个用来处理制定层的函数，然后逐层调用这个函数即可。
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]
]
confused what"{1,#,2,3}"means? > read more on how binary tree is serialized on OJ.
OJ's Binary Tree Serialization:
The serialization of a binary tree follows a level order traversal, where '#' signifies a path terminator where no node exists below.
Here's an example:
1
/
2 3
/
4
5
The above binary tree is serialized as"{1,2,3,#,#,4,#,#,5}".
/**
* Definition for binary tree
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
vector<vector<int> > zigzagLevelOrder(TreeNode* root){
vector<vector<int> >result;
for (int level = 0;; level++){
vector<int> q = ZigzagNodeAtLevel(root, level);
if (q.size() == 0){
break;
}
result.push_back(q);
}
return result;
}
vector<int> ZigzagNodeAtLevel(TreeNode* root, int level){
vector<int> res;
if (!root || level<0){
return res;
}
if (level == 0){
res.push_back(root->val);
}
// return PrintNodeAtLevel(root->left, level - 1) + PrintNodeAtLevel(root->right, level - 1);
vector<int> left_res = ZigzagNodeAtLevel(root->left, level - 1);
vector<int> right_res = ZigzagNodeAtLevel(root->right, level - 1);
if(level%2==1){
for (int i = right_res.size()-1; i>=0; i--){
res.push_back(right_res[i]);
}
for (int i = left_res.size()-1; i >=0; i--){
res.push_back(left_res[i]);
}
}else{
for (int i = left_res.size()-1; i >=0; i--){
res.push_back(left_res[i]);
}
for (int i = right_res.size()-1; i>=0; i--){
res.push_back(right_res[i]);
}
}
return res;
}
};