Given a binary tree, return thezigzag level ordertraversal 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]
]
Solution: 用deque
public List<Integer> zigZag(TreeNode root) {
if (root == null) {
return new ArrayList<Integer>();
}
Deque<TreeNode> deque = new LinkedList<>();
List<Integer> ans = new ArrayList<>();
deque.offerLast(root);
boolean reverse = false;
while (deque.size() > 0) {
int size = deque.size();
for (int i = 0; i < size; i++) {
TreeNode cur;
if (reverse) {
cur = deque.pollFirst();
if (cur.left != null) {
deque.offerLast(cur.left);
}
if (cur.right != null) {
deque.offerLast(cur.right);
}
} else {
cur = deque.pollLast();
if (cur.right != null) {
deque.offerFirst(cur.right);
}
if (cur.left != null) {
deque.offerFirst(cur.left);
}
}
ans.add(cur.key);
}
reverse = !reverse;
}
return ans;
}