利用栈实现

/*
struct TreeNode {
    int val;
    struct TreeNode *left;
    struct TreeNode *right;
    TreeNode(int x) :
            val(x), left(NULL), right(NULL) {
    }
};
*/
class Solution {
public:
    vector<vector<int> > Print(TreeNode* pRoot) {
        stack<TreeNode*> s1,s2;
        vector<vector<int>> vec;
        vector<int> v;
        int count=0;
        if(pRoot==NULL){
            return vec;
        }
        s1.push(pRoot);
        while(!s1.empty()||!s2.empty()){
            vector<int> arr;
            while(!s1.empty()){
                TreeNode* p=s1.top();
                s1.pop();
                arr.push_back(p->val);
                if(p->left){
                    s2.push(p->left);
                }
                if(p->right){
                    s2.push(p->right);
                }
                
            }
            if(arr.size()){
                vec.push_back(arr);
            }
            arr.clear();
            while(!s2.empty()){
                TreeNode *p=s2.top();
                s2.pop();
                arr.push_back(p->val);
                if(p->right) s1.push(p->right);
                if(p->left)  s1.push(p->left);
                
            }
            if(arr.size()){
                vec.push_back(arr);
            }
            arr.clear();
        }
        return vec;
    }
    
};