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：

 struct TreeNode {
     int            val;
     TreeNode*    left;
     TreeNode*    right;
     TreeNode(int x): val(x), left(NULL),right(NULL) {}
 };

 void Swap(vector<int> &ivec)
 {
     int temp = ;
     int len = ivec.size();
     for (int i = ; i < len/; i ++) {
         temp = ivec.at(i);
         ivec.at(i) = ivec.at(len--i);
         ivec.at(len--i) = temp;
     }
 }

 vector<vector <int> > zigzagLevelOrder(TreeNode *root)
 {
     vector<vector<int> > tree_vector;
     vector<int> ivec;
     queue<TreeNode *> tree_queue;
     if (NULL == root)
         return tree_vector;

     tree_queue.push(root);
     tree_queue.push(NULL);
     int nLevelCount = ;
     while (true) {
         TreeNode *pTemp = tree_queue.front();
         tree_queue.pop();
         if (pTemp == NULL) {
             if (nLevelCount% == ) { //if the num of level is odd, swap the ivec;
                 Swap(ivec);
             }
             tree_vector.push_back(ivec);
             ivec.clear();
             if (tree_queue.empty())
                 break;
             tree_queue.push(NULL);
             nLevelCount ++;
         }
         else {
             ivec.push_back(pTemp->val);
             if (pTemp->left)
                 tree_queue.push(pTemp->left);
             if (pTemp->right)
                 tree_queue.push(pTemp->right);
         }
     }
     return tree_vector;
 }