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;
}
};