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]
]

//BFS
// Use queue, and vector<TreeNode*> level. Put root into queue first. Find its queue size.For each level, 
//save to level and and put in left/right child into queue.


/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 */


class Solution {
public:
    /*
     * @param root: A Tree
     * @return: Level order a list of lists of integer
     */
    vector<vector<int>> levelOrder(TreeNode * root) {
        // write your code here
        vector<vector<int>> results;
        if (root == NULL) {
            return results;
        }
        queue<TreeNode*> q;
        q.push(root);
        while (!q.empty()) {
            vector<int> cur;
            int size = q.size();
            for (int i = 0; i < size; i++) {
                TreeNode* top = q.front();
                q.pop();
                cur.push_back(top->val);
                if (top->left != NULL) {
                    q.push(top->left);
                }
                if (top->right != NULL) {
                    q.push(top->right);
                }
            }
            results.push_back(cur);
        }
        return results;
    }
};


//DFS
// Find its height, when dfs, count depth number, and use vector[number] to save each level value.

/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 */


class Solution {
public:

     //DFS, Find height H, restore numbers to each level
    vector<vector<int>> levelOrder(TreeNode * root) {

        int height = findHight(root);
        vector<vector<int>> results(height);

        dfs(root, 0 , height, results);
        return results;
    }
    void dfs(TreeNode* root, int number, int height, vector<vector<int>>&results) {
        if (root == NULL) {
            return;
        } 
        results[number].push_back(root->val);
        dfs(root->left, number+1, height, results);
        dfs(root->right, number+1, height, results);

    }

    int findHight(TreeNode* root) {
        if (root == NULL) {
            return 0;
        } 
        int leftHight = findHight(root->left);
        int rightHight = findHight(root->right);
        return max(leftHight, rightHight) + 1;
    }
};