从上到下按层打印2叉树，同1层结点从左至右输出。每层输出1行。
2叉树的层次遍历
用队列
import java.util.ArrayList;
import java.util.*;
/*
public class TreeNode {
int val = 0;
TreeNode left = null;
TreeNode right = null;
public TreeNode(int val) {
this.val = val;
}
}
*/
public class Solution {
ArrayList<ArrayList<Integer> > Print(TreeNode pRoot) {
ArrayList<Integer> row = new ArrayList<Integer>();
ArrayList<ArrayList<Integer> > result = new ArrayList<ArrayList<Integer> >();
if(pRoot == null)
return result;
Queue<TreeNode> queue = new LinkedList<TreeNode>();
queue.offer(pRoot);
while(!queue.isEmpty()){
int size = queue.size();
row = new ArrayList<Integer>();
while((size--)>0){
TreeNode node = queue.poll();
row.add(node.val);
if(node.left!=null)
queue.offer(node.left);
if(node.right!=null)
queue.offer(node.right);
}
result.add(row);
}
return result;
}
}