leetcode 二叉树的锯齿形层次遍历[1]

https://leetcode-cn.com/submissions/detail/9721522/

给定一个二叉树，返回其节点值的锯齿形层次遍历。（即先从左往右，再从右往左进行下一层遍历，以此类推，层与层之间交替进行）。

例如：
给定二叉树 [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

返回锯齿形层次遍历如下：

[
  [3],
  [20,9],
  [15,7]
]

双栈结构

/**
 * 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> zigzagLevelOrder(TreeNode* root) {
            vector> res;
            vector temp;
            stack s1,s2;
            s1.push(root);
            if(root==NULL) return res;
            while(!(s1.empty()&&s2.empty()))
            {
                temp.clear();
                while(!s1.empty())
                {
                    TreeNode* t;
                    t=s1.top();
                    temp.push_back(t->val);
                    s1.pop();
                    if(t->left)
                        s2.push(t->left);
                    if(t->right)
                        s2.push(t->right);
                }
                if(temp.size()) res.push_back(temp);

                temp.clear();
                while(!s2.empty())
                {
                    TreeNode* t;
                    t=s2.top();
                    temp.push_back(t->val);
                    s2.pop();

                    if(t->right)
                        s1.push(t->right);
                    if(t->left)
                        s1.push(t->left);
                }
                if(temp.size()) res.push_back(temp);

            }
            return res;

        }
};