程序员面试经典--创建二叉树层结点链表

4.4问题：

给定一棵二叉树，设计一个算法，创建含有某一深度上所有节点的链表（比如，若一棵树的深度为D，则会创建出D个链表）。

思考：

1、我们可以将前序遍历稍加修改，将leve+1传入下一个递归调用。下面是使用深度优先搜索的实现代码。

public void createLevelLinkedList(Node root,ArrayList<LinkedList<Node>> lists,int level){//先序遍历--深度优先搜索法if(root == null) return;LinkedList<Node> list = null;if(lists.size() == level){list = new LinkedList<Node>();lists.add(list);}else{list = lists.get(level);}list.add(root);createLevelLinkedList(root.leftChild,lists,level + 1);createLevelLinkedList(root.rightChild,lists,level + 1);}public ArrayList<LinkedList<Node>> createLevelLinkedList(Node root){ArrayList<LinkedList<Node>> lists = new ArrayList<LinkedList<Node>>();createLevelLinkedList(root,lists,0);return lists;}

2、另一种做法是对广度有限算法稍加修改，即从根节点开始迭代，然后第2层，第3层，等等。处于第i层的时候，则表明我们已经访问过第i-1层的所有结点。也就是说，要得到i层的结点，只需直接查看i-1层结点的所有子结点即可。

public ArrayList<LinkedList<Node>> createLevelLinkedList2(Node root){//广度优先搜索法ArrayList<LinkedList<Node>> result = new ArrayList<LinkedList<Node>>();LinkedList<Node> current = new LinkedList<Node>();if(root!=null){current.add(root);}while(current.size()>0){result.add(current);//加入上一层LinkedList<Node> parents = current;//转到下一层current = new LinkedList<Node>();for(Node parent : parents){if(parent.leftChild!=null){current.add(parent.leftChild);}if(parent.rightChild!=null){current.add(parent.rightChild);}}}return result;}

完整测试程序：

import java.util.*; class Node {Node leftChild;Node rightChild;int data;Node(int newData) {//构造方法leftChild = null;rightChild = null;data = newData;}}class BinTreeTraverse{//创建的为完全二叉树。public static Node createMinimalBST(int arr[],int start,int end){if(end<start){return null;}int mid = (start+end)/2;Node n = new Node(arr[mid]);n.leftChild = createMinimalBST(arr,start,mid - 1);n.rightChild = createMinimalBST(arr,mid + 1,end);return n;}public static Node createMinimalBST(int array[]){//方法重载return createMinimalBST(array,0,array.length - 1);}public void createLevelLinkedList(Node root,ArrayList<LinkedList<Node>> lists,int level){//先序遍历--深度优先搜索法if(root == null) return;LinkedList<Node> list = null;if(lists.size() == level){list = new LinkedList<Node>();lists.add(list);}else{list = lists.get(level);}list.add(root);createLevelLinkedList(root.leftChild,lists,level + 1);createLevelLinkedList(root.rightChild,lists,level + 1);}public ArrayList<LinkedList<Node>> createLevelLinkedList(Node root){ArrayList<LinkedList<Node>> lists = new ArrayList<LinkedList<Node>>();createLevelLinkedList(root,lists,0);return lists;}public ArrayList<LinkedList<Node>> createLevelLinkedList2(Node root){//广度优先搜索法ArrayList<LinkedList<Node>> result = new ArrayList<LinkedList<Node>>();LinkedList<Node> current = new LinkedList<Node>();if(root!=null){current.add(root);}while(current.size()>0){result.add(current);//加入上一层LinkedList<Node> parents = current;//转到下一层current = new LinkedList<Node>();for(Node parent : parents){if(parent.leftChild!=null){current.add(parent.leftChild);}if(parent.rightChild!=null){current.add(parent.rightChild);}}}return result;}/** * 先序遍历 */public static void preOrderTraverse(Node node){if (node == null)return;System.out.print(node.data + " ");preOrderTraverse(node.leftChild);preOrderTraverse(node.rightChild);}/** * 中序遍历 */public static void inOrderTraverse(Node node) {if (node == null)return;inOrderTraverse(node.leftChild);System.out.print(node.data + " ");inOrderTraverse(node.rightChild);}/** * 后序遍历 */public static void postOrderTraverse(Node node) {if (node == null)return;postOrderTraverse(node.leftChild);postOrderTraverse(node.rightChild);System.out.print(node.data + " ");}}class LevellinkedList{public static void main(String[] args){int array[]=new int[9];for(int i=0;i<array.length;){array[i]=++i;}//建立二叉树以及遍历System.out.println("二叉树建立以及遍历：");BinTreeTraverse binTree = new BinTreeTraverse();Node root = binTree.createMinimalBST(array);System.out.println("先序遍历：");binTree.preOrderTraverse(root);System.out.println();System.out.println("中序遍历：");binTree.inOrderTraverse(root);System.out.println();System.out.println("后序遍历：");binTree.postOrderTraverse(root);ArrayList<LinkedList<Node>> lists = new ArrayList<LinkedList<Node>>();lists = binTree.createLevelLinkedList2(root); for(LinkedList<Node> list:lists){//增强型for循环遍历System.out.println();for(Node n: list){//增强型for循环遍历System.out.print(n.data+"->");}}System.exit(0);}}