每日一练（19）：从上到下打印二叉树

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

/**
 * Definition for a binary tree node.
 * 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) {
        queue<TreeNode*> q;
        q.push(root);
        vector<vector<int>> res;
        while (q.size()) {
            int size = q.size();
            vector<int> level;
            for (int i=0;i<size;i++) {
                TreeNode* rt = q.front();
                q.pop();
                if (!rt) {
                    continue;
                }
                level.push_back(rt->val);
                if (rt->left) {
                    q.push(rt->left);
                }
                if (rt->right) {
                    q.push(rt->right);
                }
            }
            if (level.size()!=NULL) {
                res.push_back(level);
            }
        }
        return res;
    }
};

/**
 * Definition for a binary tree node.
 * 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) {
        vector<vector<int>> res;
        dfs(root, res, 0);
        return res;
    }
    void dfs(TreeNode* root,vector<vector<int>>& res,int level)
    {
        if (!root) {
            return;
        }
        if (level >= res.size()) {
            res.emplace_back(vector<int>());
        }
        res[level].emplace_back(root->val);
        dfs(root->left, res, level+1);
        dfs(root->right, res, level+1);
    }
};