[C++]LeetCode: 31 Binary Tree Level Order Traversal

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题目：

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.

OJ's Binary Tree Serialization:

The serialization of a binary tree follows a level order traversal, where '#' signifies a path terminator where no node exists below.

Here's an example:

   1  / \ 2   3    /   4    \     5

The above binary tree is serialized as "{1,2,3,#,#,4,#,#,5}".

BFS In queue

思路：从根结点开始放入队列，每次pop时把它的左右孩子放入队列（如果存在），当该层节点pop完时插入NULL(每次都刚好在二叉树该层最右孩子放入后插入NULL).

Attention:队列辅助层序遍历，队列中插入NULL作为层与层之间的间隔，注意处理队列里最后的NULL时，不能再把它入队列以免形成死循环. 插入NULL前对队列是否非空的判断很重要（判断之后是否还有节点）。

AC Code:

/** * Definition for binary tree * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    vector<vector<int> > levelOrder(TreeNode *root) {        //输入二叉树的存储值（顺序存储，先层，再从左到右，没有的节点'#'表示）；用广度搜索BFS求二叉树每层的值。        vector<int> level;        vector<vector<int>> ret;        if(root == NULL) return ret;                queue<TreeNode*> TreeQ;        TreeQ.push(root);        TreeQ.push(NULL);   //用于区分每层                while(!TreeQ.empty())        {            TreeNode* tmp = TreeQ.front();            TreeQ.pop();                        if(tmp != NULL)            {                level.push_back(tmp->val);                if(tmp->left)   TreeQ.push(tmp->left);                if(tmp->right)  TreeQ.push(tmp->right);            }            else            {                ret.push_back(level);                //只有队列非空时才做操作，避免对树的最后一个层分隔符做死循环                if(!TreeQ.empty())                {                    level.clear();                    TreeQ.push(NULL);   //遍历完左右儿子的下一层子孙后再添加NULL                }            }        }                return ret;        }};

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