层次遍历,深刻理解树和递归。
/** * Definition for binary tree * 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>> list = new ArrayList<>(); //if(root==null) { // return null; //} bfs(0, root, list); return list; } public void bfs(int pos, TreeNode root, List<List<Integer>> list) { if(root==null) return; if(list.size() > pos) { list.get(pos).add(root.val); } else { List<Integer> l = new ArrayList<>(); l.add(root.val); list.add(l); } bfs(pos+1, root.left, list); bfs(pos+1, root.right, list); } }