LeetCode算法题目：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).
For example:
Given binary tree [3,9,20,null,null,15,7],

return its zigzag level order traversal as:

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

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分析:

此题所谓的Zig-Zag 遍历，及第一行从左到有，第二行从右到左，第三行从左到右，依次类推。此题仍然可以在层次遍历后，对结果进行处理。

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代码:

/** * 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)      {          //同层次遍历          vector<vector<int>>result;          if (!root)              return result;          queue<TreeNode*> vec;          vec.push(root);          while (vec.size() > 0) {              queue<TreeNode*> tmp_vec;              vector<int> tmp_result;              while (vec.size() > 0) {                  TreeNode* node = vec.front();                  vec.pop();                  if (node->left)                      tmp_vec.push(node->left);                  if (node->right)                      tmp_vec.push(node->right);                  tmp_result.push_back(node->val);              }              vec = tmp_vec;              result.push_back(tmp_result);          }          //对第2层、4层。。。进行翻转          for (int i = 1; i < result.size(); i += 2) {              reverse(result[i].begin(), result[i].end());          }          return result;      }  };