Leetcode 103 二叉树的锯齿层序遍历
题目定义:
给定一个二叉树,返回其节点值的锯齿形层序遍历。
(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
返回锯齿形层序遍历如下:
[
[3],
[20,9],
[15,7]
]
方式一(广度优先遍历):
class Solution {
public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
List<List<Integer>> ans = new ArrayList<>();
if(root == null)
return ans;
Queue<TreeNode> queue = new ArrayDeque<>();
queue.offer(root);
boolean leftOrRight = true;
while(!queue.isEmpty()){
LinkedList<Integer> level = new LinkedList<>();
int size = queue.size();
for(int i = 0; i < size; i++){
TreeNode node = queue.poll();
if(leftOrRight)
level.addLast(node.val);
else
level.addFirst(node.val);
if(node.left != null)
queue.offer(node.left);
if(node.right != null)
queue.offer(node.right);
}
leftOrRight = !leftOrRight;
ans.add(level);
}
return ans;
}
}
方式二(深度优先遍历):
class Solution {
public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
List<List<Integer>> ans = new ArrayList<>();
travel(ans,root,0);
return ans;
}
private void travel(List<List<Integer>> ans,TreeNode root,int level){
if(root == null)
return;
if(ans.size() <= level){
ans.add(new ArrayList<Integer>(){{add(root.val);}});
}else{
List<Integer> temp = ans.get(level);
if(level % 2 == 0)
temp.add(root.val);
else
temp.add(0,root.val);
}
travel(ans,root.left,level + 1);
travel(ans,root.right,level + 1);
}
}
参考:
https://leetcode-cn.com/problems/binary-tree-zigzag-level-order-traversal/