Binary Tree Zigzag Level Order Traversal

题目：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).

思路：层序遍历的一个小小的变种。偶数层把顺序逆置。

代码：

/**
 * 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;
        if(root == NULL)
            return result;
        vector<int> level;
        queue<TreeNode *> q;
        q.push(root);
        q.push(NULL);
        int i = 0;
        
        while(!q.empty())
        {
            TreeNode *p = q.front();
            q.pop();
            if(p == NULL)
            {
                if(i&1 == 1)
                    reverse(level.begin(), level.end());
                i++;
                result.push_back(level);
                level.clear();
                if(!q.empty())
                    q.push(NULL);
            }
            else
            {
                level.push_back(p->val);
                if(p->left != NULL)
                    q.push(p->left);
                if(p->right != NULL)
                    q.push(p->right);
            }
        }
        return result;
    }
};