题目

输入格式：

输出格式：

输入样例：

8
6 8 7 4 5 1 3 2
8 5 4 7 6 3 2 1

输出样例：

R: 1 2 3 4 5
L: 1 6 7 8 5

思路

构建二叉树

🚨难点🚨

构建过程中最容易出错的就是 确定 构建子树使用的中序、后序数组，很容易抛出数组下标越界错误。

这里简单说明一下create()这个方法：

• 作用：用中序、后序序列构建子树
• 参数：构建当前子树要用到的中序序列数组以及构建当前子树能使用的范围、后序序列数组 以及 构建当前子树能使用的范围
  
  （如图所示，构建每棵子树中后序数组使用范围不同~）

TreeNode left = create(inorder, inleft, index, postorder, poleft, poleft +(index - inleft));
TreeNode right = create(inorder, index + 1, inright, postorder, poleft +(index - inleft), poright - 1);

层序遍历二叉树

代码

import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        sc.nextLine();
        int[] inorder = new int[n];
        int[] postorder = new int[n];
        for (int i = 0; i < n; ++i) {
            inorder[i] = sc.nextInt();
        }
        sc.nextLine();
        for (int i = 0; i < n; ++i) {
            postorder[i] = sc.nextInt();
        }
        TreeNode root = create(inorder, 0, n, postorder, 0, n);

        Queue<TreeNode> que = new LinkedList<>();
        List<Integer> left = new LinkedList<>();
        List<Integer> right = new LinkedList<>();
        que.add(root);
        while (!que.isEmpty()) {
            int x = que.size();
            for (int i = 0; i < x; ++i) {
                TreeNode node = que.poll();
                if(i == 0) left.add(node.val);
                if(i == x - 1)right.add(node.val);
                if(node.left != null) que.add(node.left);
                if(node.right != null) que.add(node.right);
            }
        }
        for (int i : left) System.out.print(i + " ");
        System.out.println();
        for(int i : right) System.out.print(i + " ");
    }

    public static TreeNode create(int[] inorder, int inleft,int inright,int[] postorder, int poleft, int poright) {
        if(poright - poleft == 0)return null;
        if(poright - poleft == 1)return new TreeNode(postorder[poleft]);
        TreeNode root = new TreeNode(postorder[poright - 1]);
        int index = 0;
        int rootVal = postorder[poright - 1];
        for (int i = 0; i < inright; ++i) {
            if (inorder[i] == rootVal) {
                index = i;
                break;
            }
        }

        TreeNode left = create(inorder, inleft, index, postorder, poleft, poleft +(index - inleft));
        TreeNode right = create(inorder, index + 1, inright, postorder, poleft +(index - inleft), poright - 1);
        root.left = left;
        root.right = right;
        return root;
    }
}

class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;

    public TreeNode(int val) {
        this.val = val;
        left = null;
        right = null;
    }
}