102. Binary Tree Level Order Traversal
Given a binary tree, return the level order traversal of its nodes’ values. (ie, from left to right, level by level).
For example:
Given binary tree
For example:
Given binary tree
[3,9,20,null,null,15,7],
3
/ \
9 20
/ \
15 7
return its level order traversal as:
[ [3], [9,20], [15,7] ]
思路：题目意思为给定一个二叉树按顺序返回每一层所有节点的值。递归把每个val传入List中
代码如下：
public class BinaryTreeLevelOrderTraversal {
public List<List<Integer>> levelOrder(TreeNode root) {
List<List<Integer>> res = new ArrayList<>();
dfs(root, res, 0);
return res;
}
public static void dfs(TreeNode node, List<List<Integer>> res, int deepth) {
if (node == null) return;
if (deepth >= res.size()) {
res.add(new ArrayList<>());
}
res.get(deepth).add(node.val);
dfs(node.left, res, deepth + 1);
dfs(node.right, res, deepth + 1);
}
}