LeetCode: 103_Binary Tree Zigzag Level Order Traversal | 二叉树Zigzag层次遍历 | Medium

本题也属于层次遍历的变形，不同之处在于其遍历的方法是交替进行的，形成一个ZigZag的曲线形式，如下：

代码如下：

 1 struct TreeNode {
 2     int            val;
 3     TreeNode*    left;
 4     TreeNode*    right;
 5     TreeNode(int x): val(x), left(NULL),right(NULL) {}
 6 };
 7 
 8 void Swap(vector<int> &ivec)
 9 {
10     int temp = 0;
11     int len = ivec.size();
12     for (int i = 0; i < len/2; i ++) {
13         temp = ivec.at(i);
14         ivec.at(i) = ivec.at(len-1-i);
15         ivec.at(len-1-i) = temp;
16     }
17 }
18 
19 vector<vector <int> > zigzagLevelOrder(TreeNode *root)
20 {
21     vector<vector<int> > tree_vector;
22     vector<int> ivec;
23     queue<TreeNode *> tree_queue;
24     if (NULL == root)
25         return tree_vector;
26 
27     tree_queue.push(root);
28     tree_queue.push(NULL);
29     int nLevelCount = 1;
30     while (true) {
31         TreeNode *pTemp = tree_queue.front();
32         tree_queue.pop();
33         if (pTemp == NULL) {
34             if (nLevelCount%2 == 0) { //if the num of level is odd, swap the ivec;
35                 Swap(ivec);
36             }
37             tree_vector.push_back(ivec);
38             ivec.clear();
39             if (tree_queue.empty())
40                 break;
41             tree_queue.push(NULL);
42             nLevelCount ++;
43         }
44         else {
45             ivec.push_back(pTemp->val);
46             if (pTemp->left)
47                 tree_queue.push(pTemp->left);
48             if (pTemp->right)
49                 tree_queue.push(pTemp->right);
50         }
51     }
52     return tree_vector;
53 }