标签：Binary Level res Tree queue add null root List
Solution 1:
class Solution {
public List<List<Integer>> levelOrder(TreeNode root) {
Queue<TreeNode> queue = new LinkedList<>();
List<List<Integer>> wrapList = new LinkedList<>();
if(root == null)
return wrapList;
queue.offer(root);
while (!queue.isEmpty()) {
int levelNum = queue.size();
List<Integer> subList = new LinkedList<>();
for(int i = 0; i < levelNum; i++) {
TreeNode n = q.poll();
if(n.left != null) q.add(n.left);
if(n.right != null) q.add(n.right);
subList.add(n.val);
}
wrapList.add(subList);
}
return wrapList;
}
}
//如果要求把null节点也列出来：
public static List<List<Integer>> levelOrder(TreeNode root) {
Queue<TreeNode> queue = new LinkedList<>();
List<List<Integer>> wrapList = new LinkedList<>();
if(root == null)
return wrapList;
queue.offer(root);
while (!queue.isEmpty()) {
int levelNum = queue.size();
List<Integer> subList = new LinkedList<>();
for(int i = 0; i < levelNum; i++) {
TreeNode tn = queue.poll();
if(tn != null)
queue.offer(tn.left);
if(tn != null)
queue.offer(tn.right);
if (tn == null)
subList.add(null);
else
subList.add(tn.val);
}
wrapList.add(subList);
}
return wrapList;
}
Solution 2:
class Solution {
public List<List<Integer>> levelOrder(TreeNode root) {
List<List<Integer>> res = new ArrayList<>();
levelHelper(res, root, 0);
return res;
}
public void levelHelper(List<List<Integer>> res, TreeNode root, int height) {
if (root == null) return;
if (height >= res.size()) {
res.add(new LinkedList<>());
}
res.get(height).add(root.val);
levelHelper(res, root.left, height+1);
levelHelper(res, root.right, height+1);
}
}
标签：Binary,
Level,
res,
Tree,
queue,
add,
null,
root,
List
From： https://www.cnblogs.com/MarkLeeBYR/p/16906775.html