问题描述:

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,null,null,15,7],

    3
/ \
9 20
/ \
15 7

return its zigzag level order traversal as:

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

算法分析:和前面问题类似。

public class BinaryTreeZigzagLevelOrderTraversal
{
public List<List<Integer>> zigzagLevelOrder(TreeNode root)
{
List<Integer> list = new ArrayList<>();
List<List<Integer>> res = new ArrayList<>(); if(root == null)
{
return res;
} Stack<TreeNode> s1 = new Stack<>();
Stack<TreeNode> s2 = new Stack<>(); s1.push(root);
while(!s1.isEmpty() || !s2.isEmpty())
{
while(!s1.isEmpty())
{
TreeNode temp = s1.pop();
if(temp.left != null)
{
s2.push(temp.left);
}
if(temp.right != null)
{
s2.push(temp.right);
}
list.add(temp.val);
}
if(list.size() != 0)
res.add(list);
list = new ArrayList<>();
while(!s2.isEmpty())
{
TreeNode temp = s2.pop();
if(temp.right != null)
{
s1.push(temp.right);
}
if(temp.left != null)
{
s1.push(temp.left);
}
list.add(temp.val);
}
if(list.size() != 0)
res.add(list);
list = new ArrayList<>();
}
return res;
}
}

  

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