Binary Tree Zigzag Level Order Traversal

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本文介绍了一种算法，用于实现二叉树节点值的锯齿形层序遍历。从根节点开始，每经过一层，遍历的方向就会在从左到右和从右到左之间切换。通过递归方式实现，并展示了如何构造解决方案。

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},

return its zigzag level order traversal as:

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

/**
 * 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> > res;
	void traversal(TreeNode* root, int level)
	{
		if (0 == root) return;
		if (res.size() < level + 1)
		{
			vector<int> vec;
			vec.push_back(root->val);
			res.push_back(vec);
		}
		else if (level % 2 == 0)
			res[level].push_back(root->val);
		else
			res[level].insert(res[level].begin(), root->val);
		
		traversal(root->left, ++level);
		traversal(root->right, level);

	}
	vector<vector<int> > zigzagLevelOrder(TreeNode *root)
	{
		res.clear();
		traversal(root, 0);
		return res;
	}
};