剑指offer61 按之字形顺序打印二叉树

class Solution {
public:
    vector<vector<int> > Print(TreeNode* pRoot) {
        vector<vector<int> > result;
        vector<int> res;
        if(pRoot == NULL)
            return result;
        stack<TreeNode* > sta1;
        stack<TreeNode* > sta2;
        int next = 2;
        sta1.push(pRoot);
        while(sta1.size() != 0 || sta2.size() != 0){
            if(next % 2 == 0){
                if(sta1.empty()){
                    next++;
                    result.push_back(res);
                    res.clear();
                }
                else{
                    TreeNode* Node = sta1.top();
                    res.push_back(Node->val);
                    if(Node->left != NULL)
                        sta2.push(Node->left);
                    if(Node->right != NULL)
                        sta2.push(Node->right);
                    sta1.pop();
                }
            }
            else{
                if(sta2.empty()){
                    next++;
                    result.push_back(res);
                    res.clear();
                }
                else{
                    TreeNode* Node = sta2.top();
                    res.push_back(Node->val);
                    if(Node->right != NULL)
                        sta1.push(Node->right);
                    if(Node->left != NULL)
                        sta1.push(Node->left);
                    sta2.pop();
                }
            }
        }
        result.push_back(res);
        return result;
    }
};