【LC102】二叉树的层序遍历，在BFS中加入level观念

题目一个难点是，用BFS做，怎么一层一层输出结果。如果是dfs，递归参数中加level即可，BFS可以在遍历队列的时候加一个for循环达到同样效果，看代码：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        vector<vector<int>> ansArray;
        vector<int> ans;

        if (root == nullptr) return ansArray;
        queue<TreeNode*> q;
        q.push(root);

        while(!q.empty()){
            ans.clear();
            int qsize = q.size();
            for(int i = 0; i<qsize; i++){
                TreeNode* now = q.front();
                q.pop();
                ans.push_back(now->val);
                if (now->left != nullptr) q.push(now->left);
                if (now->right != nullptr) q.push(now->right);
            }
            ansArray.push_back(ans);
        }
        return ansArray;
    }
};