一、原題

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 {3,9,20,#,#,15,7},

    3   / /  9  20    /  /   15   71234512345
return its level order traversal as:
[  [3],  [9,20],  [15,7]]
一、中文
給定一個二叉樹，輸出它的每一層的結點。
三、舉例
上面的題目中給的就有例子，還是比較好理解的
四、思路
 使用兩個隊列的方式，隊列是先進先出的，所以我們這里使用兩個隊列，一個隊列存儲當前層的二叉樹結點，另外一個隊列來存儲下一層的結點，然后將當前層的左右子結點按照順序放入下一層中，處理完下一層之后將本層和下一層進行一個互換的操作。
五、程序
package code;import java.util.Deque;import java.util.Iterator;import java.util.LinkedList;//class TreeNode{//	int val;//	TreeNode left;//	TreeNode right;//	TreeNode(int x){//		val = x;//	}//}public class LeetCode51{	public static void main(String args[]){		TreeNode n1 = new TreeNode(1);		TreeNode n2 = new TreeNode(2);		TreeNode n3 = new TreeNode(2);		TreeNode n4 = new TreeNode(3);		TreeNode n5 = new TreeNode(4);		TreeNode n6 = new TreeNode(3);		TreeNode n7 = new TreeNode(4);				n1.left = n2;		n1.right = n3;				n2.left = n4;		n2.right = n5;				n3.left = n7;		n3.right = n6;				LinkedList<LinkedList<Integer>> res = levelOrder(n1);				Iterator<LinkedList<Integer>> it1 = res.iterator();		while(it1.hasNext()){			Iterator<Integer> it2 = it1.next().iterator();			while(it2.hasNext()){				System.out.PRint(it2.next()+" ");			}			System.out.println();		}	}	//第一種使用遞歸的方式	public static LinkedList<LinkedList<Integer>> levelOrder(TreeNode root) {		LinkedList<LinkedList<Integer>> res = new LinkedList<>();				if(root == null){			return res;		}		        Deque<TreeNode> cur = new LinkedList<>();        Deque<TreeNode> sub = new LinkedList<>();        Deque<TreeNode> exc;                cur.addLast(root);        TreeNode node;                while(!cur.isEmpty()){        	LinkedList<Integer> temp = new LinkedList<>();        	        	while(!cur.isEmpty()){        		node = cur.removeFirst();        		temp.add(node.val);        		        		if(node.left != null){        			sub.addLast(node.left);	        		}        		        		if(node.right != null){        			sub.addLast(node.right);	        		}        		        	}        	    		exc = cur;            cur = sub;            sub = exc;            res.add(temp);        }        return res;	}	}	----------------------------------------output-------------------------------------------
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