Binary Tree Zigzag Level Order Traversal

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},

    3
   / \
  9  20
    /  \
   15   7

 

return its zigzag level order traversal as:

[
  [3],
  [20,9],
  [15,7]
]
分析：这道与level order traversal不同的地方在于相邻层元素的访问方向是相反的，我们可以用一个bool变量来标记访问的方向，代码如下：
 1 class Solution {
 2 public:
 3     vector<vector<int> > zigzagLevelOrder(TreeNode *root) {
 4         vector<vector<int> > result;
 5         if(root == NULL) return result;
 6         
 7         bool direction = false;//false indicates from left to right
 8         vector<TreeNode *> prev(1, root);
 9         
10         while(!prev.empty()){
11             vector<int> level;
12             vector<TreeNode *> cur;
13             for(auto i = prev.begin(); i != prev.end(); i++){
14                 level.push_back((*i)->val);
15                 if((*i)->left) cur.push_back((*i)->left);
16                 if((*i)->right) cur.push_back((*i)->right);
17             }
18             if(direction)
19                 reverse(level.begin(), level.end());
20             result.push_back(level);
21             prev = cur;
22             direction ^= true; 
23         }
24         
25         return result;
26     }
27 };

 

 

 

posted on 2014-08-16 00:15  Ryan-Xing  阅读(112)  评论(0)  编辑  收藏  举报