[Leetcode]103. 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，先从左到右遍历，接着一层从右到左遍历，依次交替进行。通过递归方式实现，并附带C++代码示例。

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],

return its zigzag level order traversal as:

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

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