leetcode || 102、Binary Tree Level Order Traversal

problem：

Given a binary tree, return the level order traversal of its nodes‘ values. (ie, from left to right, level by level).

For example:

Given binary tree {3,9,20,#,#,15,7},

    3
   /   9  20
    /     15   7

return its level order traversal as:

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

confused what "{1,#,2,3}" means? >
read more on how binary tree is serialized on OJ.

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题意：层序遍历二叉树，每层要分开

thinking：

（1）普通的二叉树层序遍历算法每层是没有分开的，这道题要求分开。

（2）采用广度优先搜索（BFS）的思想

（3）采用两个queue，递归实现每层分开的层序遍历，很经典

code：

class Solution {
  private:
      vector<vector<int> > ret;
  public:
      vector<vector<int> > levelOrder(TreeNode *root) {
          ret.clear();
          if(root==NULL)
              return ret;
          queue<TreeNode *> tmp_queue;
          tmp_queue.push(root);
          level_order(tmp_queue);
          return ret;
      }
      void level_order(queue<TreeNode *> queue1)
      {
          if(queue1.empty())
              return;
          vector<int> array;
          queue<TreeNode *> queue2;
          while(!queue1.empty())
          {
              TreeNode *tmp=queue1.front();
              array.push_back(tmp->val);
              queue1.pop();
              if(tmp->left!=NULL)
                  queue2.push(tmp->left);
              if(tmp->right!=NULL)
                  queue2.push(tmp->right);
          }
          ret.push_back(array);
          level_order(queue2);
      }
  };