Question Definition
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]
]
Java Solution
public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
List<List<Integer>> result = new LinkedList<>();
if(root == null) return result;
Deque<TreeNode> stack1 = new ArrayDeque<>();
Deque<TreeNode> stack2 = new ArrayDeque<>();
stack1.push(root);
while(!stack1.isEmpty() || !stack2.isEmpty()){
List<Integer> level = new LinkedList<>();
if(!stack1.isEmpty()){
while(!stack1.isEmpty()){
TreeNode node = stack1.pop();
if(node.left != null)
stack2.push(node.left);
if(node.right != null)
stack2.push(node.right);
level.add(node.val);
}
}else{
while(!stack2.isEmpty()){
TreeNode node = stack2.pop();
if(node.right != null)
stack1.push(node.right);
if(node.left != null)
stack1.push(node.left);
level.add(node.val);
}
}
result.add(level);
}
return result;
}
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