107. Binary Tree Level Order Traversal II LeetCode

题意：二叉树的层序遍历。
题解：BFS即可。

class Solution {
public:
    vector<vector<int>> levelOrderBottom(TreeNode* root) {
        queue<TreeNode*> q;
        vector<vector<int>> ans;
        while(!q.empty()) q.pop();
        ans.clear();
        if(root == NULL) return ans;
        q.push(root);
        while(!q.empty())
        {
            int n = q.size();
            vector<int> v;
            for(int i = 0; i < n; i++)
            {
                TreeNode* now = q.front();
                v.push_back(now->val);
                q.pop();
                if(now->left != NULL) q.push(now->left);
                if(now->right != NULL) q.push(now->right);
            }
            ans.push_back(v);
        }
        vector<vector<int>> anss;
        int n = ans.size();
        for(int i = n - 1; i >= 0; i--)
        {
            anss.push_back(ans[i]);
        }
        return anss;
    }
};