102.二叉树的层序遍历

使用一个队列来存放每一层的结点，时间复杂度 O ( n ) O(n) O(n) ，空间复杂度 O ( n ) O(n) O(n) ：

class Solution {
public:
    vector> levelOrder(TreeNode* root) {
        queue q;
        vector> ans;
        if(root == nullptr)
            return ans;
        q.push(root);
        int flag = 1;
        while(flag != 0) {
            vector tmp;
            int ct = 0;
            for(int i = 0; i < flag; ++i) {
                tmp.push_back(q.front()->val);
                if(q.front()->left) {
                    q.push(q.front()->left);
                    ct++;
                }
                if(q.front()->right) {
                    q.push(q.front()->right);
                    ct++;
                }
                q.pop();
            }
            flag = ct;
            ans.push_back(tmp);
        }
        return ans;
    }
};