LeetCode: 102_Binary Tree Level Order Traversal | 二叉树自顶向下的层次遍历 | Easy

题目：Binay Tree Level Order Traversal

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},

如下一棵树

转换之后需要输出这样的形式：

如下，见代码：

struct TreeNode {
    int        val;
    TreeNode*    left;
    TreeNode*    right;
    TreeNode():val(0),left(NULL),right(NULL) {}
    TreeNode(int x): val(x), left(NULL),right(NULL) {}
};


vector<vector<int> > levelOrderTraversal(TreeNode *root) //非递归的中序遍历(用栈实现)
{

    queue<TreeNode *> tree_queue;
    vector<vector<int> > tree_vector;
    vector<int> svector;

    if (NULL == root) {
        return tree_vector;
    }
    TreeNode *pTemp = root;
    tree_queue.push(root);
    tree_queue.push(NULL); //the end of one level.

    while (true) {
        TreeNode *pTemp = tree_queue.front();
        tree_queue.pop();

        if (!pTemp) { //get the null, put vector<> to vector<vector<>>
            tree_vector.push_back(svector);
            svector.clear();
            if (tree_queue.empty())
                break;
            tree_queue.push(NULL);
        }
        else {
            svector.push_back(pTemp->val);
            if (pTemp->left) 
                tree_queue.push(pTemp->left);
            if (pTemp->right)
                tree_queue.push(pTemp->right);
        }
    }
    return tree_vector;
}