Binary Tree Zigzag Level Order Traversal
Description
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] ] Show Company Tags Show Tags Show Similar Problems
Hint
Train of Thought
I use a queue to implement BFS. Each time when I poll a node, I add this node value to level. I use a variable zigzag to indicate whether add from left to right or right to left. If zigzag == false, it is from left to right; if zigzag == true, it is from right to left. And from right to left just need to use ArrayList.add(0, value) method
Code
List<List<Integer>> res = new ArrayList<>();
if (root == null) return res;
Queue<TreeNode> queue = new LinkedList<>();
queue.add(root);
boolean zigzag = false;
while (!queue.isEmpty()) {
List<Integer> level = new ArrayList<>();
int cnt = queue.size();
for (int i = 0; i < cnt; i++) {
TreeNode node = queue.poll();
if (zigzag) {
level.add(0, node.val);
}
else {
level.add(node.val);
}
if (node.left != null) {
queue.add(node.left);
}
if (node.right != null) {
queue.add(node.right);
}
}
res.add(level);
zigzag = !zigzag;
}
return res;