Leetcode中级 二叉树的锯齿形层次遍历C++

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]
]

思路：利用两个栈即可解决

class Solution {
public:
    vector<vector<int>> zigzagLevelOrder(TreeNode* root) {
        vector<vector<int>> res;
        if(!root) return res;
        stack<TreeNode*> s1,s2;
        s1.push(root);
        while(!s1.empty() || !s2.empty())
        {
            vector<int> tmp;
            if(!s1.empty())
            {
                while(!s1.empty())
                {
                    TreeNode* cur = s1.top();
                    s1.pop();
                    if(cur->left)
                        s2.push(cur->left);
                    if(cur->right)
                        s2.push(cur->right);
                    tmp.push_back(cur->val);
                }
            }
            else
            {
                while(!s2.empty())
                {
                    TreeNode* cur = s2.top();
                    s2.pop();
                    if(cur->right)
                        s1.push(cur->right);
                    if(cur->left)
                        s1.push(cur->left);
                    tmp.push_back(cur->val);
                }
            }
            res.push_back(tmp);
        }
        return res;
    }
};