LeetCode

    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);
	}
}