leetcode z形遍历二叉树

key:  use two stack s1,s2 

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<vector<int> > zigzagLevelOrder(TreeNode *root) {
        vector<vector<int>> res;
        if(!root) return res;
        stack<TreeNode*> s1,s2;
        s1.push(root);
        vector<int> temp;
        while(!s1.empty() || !s2.empty()){
            
            while(!s1.empty()){
                TreeNode* cur = s1.top();
                temp.push_back(cur->val);
                s1.pop();
                if(cur->left) s2.push(cur->left);
                if(cur->right) s2.push(cur->right);
                
            }
            if(!temp.empty()) {
               res.push_back(temp);
               temp.clear();
            }
            while(!s2.empty()){
                TreeNode* cur = s2.top();
                temp.push_back(cur->val);
                s2.pop();
                if(cur->right) s1.push(cur->right);
                if(cur->left) s1.push(cur->left);
            }
            if(!temp.empty()) {
                res.push_back(temp);
                temp.clear();
            }
        }
        return res;
        
    }
};