[leetcode]剑指 Offer 32 - III. 从上到下打印二叉树 III BFS二叉树

/**
 * 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>> ret;
        if (root == NULL) return ret;
        queue < TreeNode * > que;
        vector<int> t;
        int pre = 1;
        que.push(root);
        int cnt = 0;
        int flag = 1;
        while (que.size()) {
            TreeNode *node = que.front();
            que.pop();
            pre--;
            t.push_back(node->val);
            if (node->right != NULL) {
                que.push(node->right);
                cnt++;
            }
            if (node->left != NULL) {
                que.push(node->left);
                cnt++;
            }
            if (pre == 0) {
                pre = cnt;
                cnt = 0;
                if (flag == -1)
                    ret.push_back(t);
                else {
                    reverse(t.begin(), t.end());
                    ret.push_back(t);
                }
                t.clear();
                flag *= -1;
            }
        }
        return ret;
    }
};