二叉树“之”字型遍历

思路：使用两个栈来实现，先将根节点入栈s1，借用层序遍历的思想，将根节点弹出栈s1时，将该节点的孩子存入另一给栈s2中。
注意，后续入栈时，s1是先将右孩子入栈，再将左孩子入栈，这样才能保证从s1弹出时是正序的。

/*
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) {
        vector<vector<int>> res;
        if(pRoot==NULL)  return res;
        TreeNode* root;
        stack<TreeNode *> s1;
        stack<TreeNode *> s2;
        s1.push(pRoot);
        while(!s1.empty()||!s2.empty()){
            vector<int> temp;
            while(!s1.empty()){
                root=s1.top();
                s1.pop();
                if(root->val)
                    temp.push_back(root->val);
                if(root->left)  s2.push(root->left);
                if(root->right)  s2.push(root->right);

            }
            if(temp.size()){
                res.push_back(temp);
                temp.clear();
            }

            while(!s2.empty()){
                root=s2.top();
                s2.pop();
                if(root->val)
                    temp.push_back(root->val);
                if(root->right)  s1.push(root->right);
                if(root->left)  s1.push(root->left);
            }

            if(temp.size()){
                res.push_back(temp);
            }
        }
        return res;


    }

};