03-树2 List Leaves（java）

题目

Given a tree, you are supposed to list all the leaves in the order of top down, and left to right.

Input Specification:

Each input file contains one test case. For each case, the first line gives a positive integer $N$ ($\le 10$) which is the total number of nodes in the tree -- and hence the nodes are numbered from 0 to $N-1$. Then $N$ lines follow, each corresponds to a node, and gives the indices of the left and right children of the node. If the child does not exist, a "-" will be put at the position. Any pair of children are separated by a space.

Output Specification:

For each test case, print in one line all the leaves' indices in the order of top down, and left to right. There must be exactly one space between any adjacent numbers, and no extra space at the end of the line.

Sample Input:

81 -- -0 -2 7- -- -5 -4 6

Sample Output:

4 1 5

题目很简单，就是建树，并按层序层序遍历的顺序输出叶子节点。

建立一个队列，先入队根节点，然后出队，入根节点的子节点，接下来每次出队都将它的子节点入队。只要在出队的时候判断是否为叶子节点并输出即可

package week32;import java.util.*;class TreeNode{public int data;public int left;public int right;public int check=0;}public class Main {private static Scanner in;//寻找根节点static int findRoot(TreeNode[] t){int root=0;for(int i=0;i<t.length;i++){if(t[i].check==0)root=i;}return root;}//判断是否叶子节点static boolean leaves(TreeNode node){if(node.left==-1&&node.right==-1){return true;}else{return false;}}//建树public static TreeNode[] buildTree(){int num=Integer.parseInt(in.nextLine());//注意不能用nextintTreeNode[] tree=new TreeNode[num];for(int i=0;i<num;i++){TreeNode a = new TreeNode();String s=in.nextLine();if(s.substring(0, 1).equals("-")){a.left=-1;}else{a.left=Integer.parseInt((s.substring(0, 1)));}if(s.substring(2,3).equals("-")){a.right=-1;}else{a.right=Integer.parseInt((s.substring(2, 3)));}//输入为-则设为-1.否则输入数值a.data=i;tree[i]=a;}for(int i=0;i<num;i++){if(tree[i].left!=-1){tree[(tree[i].left)].check=-1;}if(tree[i].right!=-1){tree[(tree[i].right)].check=-1;}}return tree;}////层序遍历入队，输出时判断即可public static void cengXu(TreeNode[] tree){Queue<TreeNode> queue=new LinkedList<TreeNode>();int root=findRoot(tree);queue.offer(tree[root]);int l=tree.length;boolean first=true;while(l!=0){TreeNode t=queue.poll();if(leaves(t)){if(first){first=false;System.out.print(t.data);}elseSystem.out.print(" "+t.data);}else{if(t.left!=-1)queue.offer(tree[t.left]);if(t.right!=-1)queue.offer(tree[t.right]);}l--;}}public static void main(String[] args) {// TODO Auto-generated method stubin=new Scanner(System.in);TreeNode[] tree=buildTree();cengXu(tree);}}

写的有些啰嗦，如果有人看了能请改进意见，谢谢