Binary Tree Zigzag Level Order Traversal

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]
]

BFS,宽度查找法。把同一级别的宽度记下来。訪问的同一时候把下一层弄到队列里面去。经常使用遍历方法!

class Solution {
public:
  vector<vector<int>> zigzagLevelOrder(TreeNode* root) 
	{
		vector<vector<int>> levelOrderStore;
		if (root==NULL)
			return levelOrderStore;
		queue<TreeNode *>q;
		stack<TreeNode *>rightout;
		q.push(root);
		rightout.push(root);
		int count=1;
		int level=0;
		vector<int> tmp(0);
		int flag=0;//begin  right->left->right
		while (!q.empty())//q.root
		{
			tmp.clear();
			level=0;
			for (int i=0;i<count;i++)//level
			{

				root=q.front(); //every root.
				q.pop();
			
				if (flag%2==0)
				{
					tmp.push_back(root->val);
				}
				else
				{
					tmp.push_back(rightout.top()->val); //every root.
						rightout.pop();//
				}
			
				if (root->left!=NULL)
				{
					q.push(root->left);
					if (flag%2==0)
					{
						rightout.push(root->left); //
					}
					++level;
				}
				if (root->right!=NULL)
				{
					q.push(root->right);
					if (flag%2==0)
					rightout.push(root->right);
					++level;
				}
			}
			count=level;
			levelOrderStore.push_back(tmp);
			flag++;
		}

		return levelOrderStore;
	}

};