Binary Tree Level Order Traversal II

層序遍歷，先用一個棧把結點分層塞進去，再逐個出棧：

/**
 * 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> >re;
        stack<vector<TreeNode*> > SaveTree;
        if(root==NULL)
        return re;
        vector<TreeNode*> first;
        first.push_back(root);
        SaveTree.push(first);
        while(!(SaveTree.top()).empty())
        {
            vector<TreeNode*> next;
            vector<TreeNode*> & pre=SaveTree.top();
            for(int i=0;i<pre.size();i++)
            {
                if(pre[i]->left!=NULL)
                next.push_back(pre[i]->left);
                if(pre[i]->right!=NULL)
                next.push_back(pre[i]->right);
            }
            SaveTree.push(next);
        }
        SaveTree.pop();
        while(!SaveTree.empty())
        {
            vector<TreeNode*> & temp=SaveTree.top();
            vector<int> pushIn;
            for(int i=0;i<temp.size();i++)
            {
                pushIn.push_back(temp[i]->val);
            }
            re.push_back(pushIn);
            SaveTree.pop();
        }
        return re;
        
    }
};