剑指offer 分行从上到下打印二叉树

题目：

从上到下按层打印二叉树，同一层的节点按照从左到右的顺序打印，每一层打印到一行。

/*
struct TreeNode {
    int val;
    struct TreeNode *left;
    struct TreeNode *right;
    TreeNode(int x) :
            val(x), left(NULL), right(NULL) {
    }
};*/
class Solution {
public:
    vector<vector<int> > Print(TreeNode* root) {
        vector<vector<int> > res;
        vector<int> v;
        if (root == NULL)
            return res;
        TreeNode* temp;
        queue<TreeNode*> q;
        q.push(root);
        while(!q.empty()) {
            v.clear();
            int len = q.size(); //队列中的元素个数即为当前行的所有元素
            v.resize(len, 0);
            for (int i = 0; i < len; i++) {
                temp = q.front();
                q.pop();
                v[i] = temp->val;
                if (temp->left != NULL)
                    q.push(temp->left);
                if (temp->right != NULL)
                    q.push(temp->right);
            }
            res.push_back(v);
        }
    }
};