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;    }};