求二叉树的层序遍历

求二叉树的层序遍历（牛客网）

题目：给定一个二叉树，返回该二叉树层序遍历的结果，（从左到右，一层一层地遍历）

例如：
给定的二叉树是{3,9,20,#,#,15,7},该二叉树层序遍历的结果是
[
[3],
[9,20],
[15,7]
]

class Solution {
     
public:
    /**
     * 
     * @param root TreeNode类 
     * @return int整型vector>
     */
    vector<vector<int> > levelOrder(TreeNode* root) {
     
        // write code here
        vector<vector<int>> res;
        if (root == NULL)
            return res;
        queue<TreeNode *> tree;
        tree.push(root);
        int curNum = 1, nextNum = 0;
        vector<int> cow;
        while (!tree.empty()) {
     
            TreeNode *temp = tree.front();
            tree.pop();
            curNum--;
            cow.push_back(temp->val);
            if (temp->left) {
     
                tree.push(temp->left);
                nextNum++;
            }
            if (temp->right) {
     
                tree.push(temp->right);
                nextNum++;
            }
            if (curNum == 0) {
     
                res.push_back(cow);
                curNum = nextNum;
                nextNum = 0;
                cow.erase(cow.begin(), cow.end());
            }
        }
        return res;
    }
};