题目描述：

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]
]

思路分析：层序遍历二叉树。设置一个标志变量r来表示当前层是奇数层还是偶数层。当处于偶数数层时，将当前层得到的序列反转后再插入到层序遍历的序列中，奇数层则直接插入。层序遍历结束后即得到结果。

代码：

vector<vector<int> > Solution::zigzagLevelOrder(TreeNode *root)
{
    vector<vector<int> > result;
    vector<int> sequence;
    bool r = false;
    TreeNode * flag = new TreeNode(0);
    queue<TreeNode *> treenode_queue;
    if(root == NULL)
        return result;
    treenode_queue.push(root);
    treenode_queue.push(flag);
    while(treenode_queue.size() >= 1)
    {
        TreeNode * node = treenode_queue.front();
        treenode_queue.pop();
        if(node == flag)
        {
            if(r)
            {
                reverse(sequence.begin(),sequence.end());
            }
            r = !r;
            result.push_back(sequence);
            sequence.clear();
            if(treenode_queue.empty())
                break;
            treenode_queue.push(flag);
        }
        else
        {
            sequence.push_back(node->val);
            if(node->left != NULL)
                treenode_queue.push(node->left);
            if(node->right != NULL)
                treenode_queue.push(node->right);
        }
    }
    return result;
}

LeetCode:Binary Tree Zigzag Level Order Traversal