107. Binary Tree Level Order Traversal II

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */

static int wing=[]()
{
    std::ios::sync_with_stdio(false);
    cin.tie(NULL);
    return 0;
}();

class Solution 
{
public:
    vector<vector<int>> levelOrderBottom(TreeNode* root) 
    {
        queue<TreeNode*> q;
        vector<vector<int>> vvint;
        vector<int> vint;
        if(root==NULL)
            return vvint;
        q.push(root);
        while(!q.empty())
        {
            for(int i=0,n=q.size();i<n;i++)
            {
                
                TreeNode* p=q.front();
                q.pop();
                vint.push_back(p->val);
                if(p->left!=NULL)
                    q.push(p->left);
                if(p->right!=NULL)
                    q.push(p->right);                
            }
            vvint.push_back(vint);
            vint.clear();
        }
        reverse(vvint.begin(),vvint.end());
        vector<vector<int>> &res=vvint;
        return res;
    }
};

用层次遍历，把每个元素放入vector里面，扫完一层就把vector放入vvint，扫描完整个树之后，将vvint逆置返回即可。这里最好返回引用。