/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public List<List<Integer>> levelOrder(TreeNode root) {
List<List<Integer>> result = new ArrayList<>();
levelOrderDFS(root, result, 0);
return result;
}
private void levelOrderDFS(TreeNode root, List<List<Integer>> result, int level) {
if (root == null) {
return;
}
if (result.size() < level + 1) {
List<Integer> list = new ArrayList<>();
result.add(list);
}
result.get(level).add(root.val);
levelOrderDFS(root.left, result, level + 1);
levelOrderDFS(root.right, result, level + 1);
}
}
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
/*
1. BFS 每次explored一层,for循环将一层的节点的子节点都放入queue,每次for循环结束level++
2. BFS 将(TreeNode, Level)存入queue
3. DFS 递归
func(node, height)
result[height].add(node)
func(node.left, heigh + 1)
func(node.right, heigh + 1)
*/
public class Solution {
public List<List<Integer>> levelOrder(TreeNode root) {
List<List<Integer>> res = new ArrayList<List<Integer>>();
if (root == null) return res;
Queue<TreeNode> queue = new LinkedList<TreeNode>();
queue.offer(root);
while(!queue.isEmpty()) {
int curLen = queue.size();
List<Integer> level = new ArrayList<Integer>();
for(int i = 0; i < curLen; i++) {
TreeNode temp = queue.poll();
level.add(temp.val);
if(temp.left != null) queue.offer(temp.left);
if(temp.right != null) queue.offer(temp.right);
}
res.add(level);
}
return res;
}
}