leetcode_107

/**
 * 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>> levelOrderBottom(TreeNode* root) {
        vector<vector<int>> ans;
        if(!root)
            return ans;
        vector<int> root_vec;
        root_vec.push_back(root->val);
        ans.push_back(root_vec);
        //用队列存节点，每次把节点都弹空，再压入下一层的节点
        queue<TreeNode*> que;
        que.push(root);
        while(!que.empty())//每次执行这个循环代表进入下一层
        {
            vector<int> tmp_vec;
            vector<TreeNode*> tmp_node;
            while(!que.empty())//计算其子节点
            {
                TreeNode* tmp=que.front();
                que.pop();
                if(tmp->left)
                {
                    tmp_vec.push_back(tmp->left->val);
                    tmp_node.push_back(tmp->left);
                }
                if(tmp->right)
                {
                    tmp_vec.push_back(tmp->right->val);
                    tmp_node.push_back(tmp->right);
                }
                
            }
            //cout<<tmp_node.size()<<endl;
            if(!tmp_vec.empty())
                ans.push_back(tmp_vec);
            for(int i=0;i<tmp_node.size();i++)
                que.push(tmp_node[i]);
        }
        vector<vector<int>> vec_ans;
        for(int i=ans.size()-1;i>=0;i--)
            vec_ans.push_back(ans[i]);
        return vec_ans;
    }
};