[Leetcode] Binary Tree Zigzag Level Order Traversal

Given a binary tree, return the zigzag level order traversal of its nodes' values. (ie, from left to right, then right to left for the
next level and alternate between).

For example:

Given binary tree

{3,9,20,#,#,15,7}

,

3
/ \
9  20
/  \
15   7

return its zigzag level order traversal as:

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

两个栈搞定

class Solution{
public:
vector<vector<int> > zigzagLevelOrder(TreeNode *root)
{
stack<TreeNode*> level1;
stack<TreeNode*> level2;
vector<vector<int> > result;
vector<int> levelNum;
if(root==NULL) return result;

level1.push(root);

while(!level1.empty()||!level2.empty())
{
while(!level1.empty())
{
TreeNode *topNode = level1.top();
if(topNode->left) level2.push(topNode->left);
if(topNode->right) level2.push(topNode->right);
levelNum.push_back(topNode->val);
level1.pop();
if(level1.empty())
{
result.push_back(levelNum);
levelNum.clear();
}
}

while(!level2.empty())
{
TreeNode *topNode = level2.top();
if(topNode->right) level1.push(topNode->right);
if(topNode->left) level1.push(topNode->left);
levelNum.push_back(topNode->val);
level2.pop();
if(level2.empty())
{
result.push_back(levelNum);
levelNum.clear();
}
}

}

return result;
}
};

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