從上到下按層列印二叉樹，同一層結點從左至右輸出。每一層輸出一行。

思路：定義一個佇列，遍歷二叉樹，並將結點入佇列，每入一層處理一層，保證一層一層地放入陣列中。

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<ArrayList<Integer>> ret = new ArrayList<ArrayList<Integer>>();
        ArrayList<Integer> tmp = new ArrayList<Integer>();
        LinkedList<TreeNode> q = new
 
 LinkedList<TreeNode>();
        if(pRoot == null)
            return ret;
        q.add(pRoot);
        int now = 1, next = 0;
        while(!q.isEmpty()) {
            TreeNode t = q.remove();
            now--;
            tmp.add(t.val);
            if(t.left != null) {
                q.add(t.left
 
);
                next++;
            }
            if(t.right != null) {
                q.add(t.right);
                next++;
            }
            if(now == 0) {
                ret.add(new ArrayList<Integer>(tmp));
                tmp.clear();
                now = next;
                next = 0;
            }
        }
        return ret;
    }

}