LeetCode-103-Binary Tree Zigzag Level Order Traversal

算法描述：

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,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

return its zigzag level order traversal as:

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

解题思路：与水平访问一样，用队列实现。区别在于加入一个判断，在偶数行反向插入数值就行。

    vector<vector<int>> zigzagLevelOrder(TreeNode* root) {
        vector<vector<int>> results;
        helper(root, results,0);
        return results;
    }
    
    void helper(TreeNode* root, vector<vector<int>>& results, int level){
        if(root ==nullptr) return;
        if(results.size() <= level){
            vector<int> temp;
            results.push_back(temp);
        }
        if(level%2==0){
            results[level].push_back(root->val);
        }else{
            results[level].insert(results[level].begin(),root->val);
        }

        helper(root->left,results,level+1);
        helper(root->right,results,level+1);

    }