给定一个二叉树,返回其节点值的锯齿形层次遍历。(即先从左往右,再从右往左进行下一层遍历,以此类推,层与层之间交替进行)。
例如:
给定二叉树 [3,9,20,null,null,15,7],
3
/ \
9 20
/ \
15 7
返回锯齿形层次遍历如下:
[
[3],
[20,9],
[15,7]
]
java代码
public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
List<List<Integer>> result = new LinkedList<List<Integer>>();
List<Integer> temList = new LinkedList<Integer>();
if (root == null) {
return result;
}
Queue<TreeNode> queue = new LinkedList<TreeNode>();
((LinkedList<TreeNode>) queue).add(root);
int num = 1;
int next = 0;
int flag = 0;
while (!queue.isEmpty()) {
TreeNode now = queue.poll();
num--;
if(result.size()%2==0){
temList.add(now.val);
}else {
((LinkedList<Integer>) temList).addFirst(now.val);
}
if (now.left != null) {
((LinkedList<TreeNode>) queue).add(now.left);
next++;
}
if (now.right != null) {
((LinkedList<TreeNode>) queue).add(now.right);
next++;
}
if (num == 0) {
num = next;
next = 0;
((LinkedList<List<Integer>>) result).add(temList);
temList = new LinkedList<Integer>();
}
}
return result;
}