Binary Tree Zigzag Level Order Traversal

 方法一：递归的方法，使用一个flag记录不同层次的遍历顺序，这里需要注意使用vector中insert操作

/**
 * 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>> zigzagLevelOrder(TreeNode* root) {
        vector<vector<int>> result;
        
        //true reparesent traverse from left to right
        traverse(root, 1, result, true);
        
        return result;
    }
    
    void traverse(TreeNode * root, int level, vector<vector<int>> &result, bool flag)
    {
        if(root == nullptr)
            return;
            
        if(result.size() < level)
            result.push_back(vector<int>());
        
        if(flag)
            result[level-1].insert(result[level-1].begin(), root->val);
        else
            result[level-1].push_back(root->val);
        
        
        if(root->right)
            traverse(root->right, level+1, result, !flag);
        if(root->left)
            traverse(root->left, level+1, result, !flag);
    }
};

方法二：非递归方法，使用队列数据结构

/**
 * 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>> zigzagLevelOrder(TreeNode* root) {
        vector<vector<int>> result;
        if(!root)
            return result;
            
        queue<TreeNode *> que;
        
        que.push(root);
        //true represent traverse from left to right
        bool flag = true;
        while(!que.empty())
        {
            vector<int> _result;
            vector<TreeNode *> tmp;
            while(!que.empty())
            {
                _result.push_back(que.front()->val);
                tmp.push_back(que.front());
                que.pop();
            }
            
            result.push_back(_result);
            
            for(int i=tmp.size() - 1; i >= 0; --i)
            {
                if(flag)
                {
                    if(tmp[i]->right)
                        que.push(tmp[i]->right);
                    if(tmp[i]->left)
                        que.push(tmp[i]->left);
                }
                else
                {
                    if(tmp[i]->left)
                        que.push(tmp[i]->left);
                    if(tmp[i]->right)
                        que.push(tmp[i]->right);
                }
            }
            
            flag = !flag;
        }
        
        return result;
    }

};