[PAT] 1020 Tree Traversals （25 分）Java

Suppose that all the keys in a binary tree are distinct positive integers. Given the postorder and inorder traversal sequences, you are supposed to output the level order traversal sequence of the corresponding binary tree.

Input Specification:

Each input file contains one test case. For each case, the first line gives a positive integer N (≤30), the total number of nodes in the binary tree. The second line gives the postorder sequence and the third line gives the inorder sequence. All the numbers in a line are separated by a space.

Output Specification:

For each test case, print in one line the level order traversal sequence of the corresponding binary tree. All the numbers in a line must be separated by exactly one space, and there must be no extra space at the end of the line.

Sample Input:

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

Sample Output:

4 1 6 3 5 7 2

package pattest;

import java.util.LinkedList;
import java.util.Scanner;

/**
 * @Auther: Xingzheng Wang
 * @Date: 2019/2/28 15:52
 * @Description: pattest
 * @Version: 1.0
 */
public class PAT1020 {
    static Scanner scanner = new Scanner(System.in);

    static class Node {
        int value;
        Node left;
        Node right;

        Node(int value) {
            this.value = value;
        }

        @Override
        public String toString() {
            return "Node{" + "value=" + value + ", left=" + left + ", right=" + right + '}';
        }
    }

    private static Node bulidTree() {
        int size = scanner.nextInt();
        int[] postOrder = new int[size];
        int[] inOrder = new int[size];
        for (int i = 0; i < size; i++) {
            postOrder[i] = scanner.nextInt();
        }
        for (int i = 0; i < size; i++) {
            inOrder[i] = scanner.nextInt();
        }
        Node root = build(postOrder, inOrder, 0, size - 1, 0, size - 1);
        return root;
    }

    private static Node build(int[] postOrder, int[] inOrder, int postStart, int postEnd, int inStart, int inEnd) {
        if (postStart > postEnd) {
            return null;
        }
        if (postStart == postEnd) {
            return new Node(postOrder[postStart]);
        }
        int root = postOrder[postEnd--];

        //find root in inOrder
        int inIndex = -1;
        for (int i = inStart; i <= inEnd; i++) {
            if (root == inOrder[i]) {
                inIndex = i;
                break;
            }
        }
        //recursion build
        int leftSize = inIndex - inStart;
        int rightSize = inEnd - inIndex;
        Node rootNode = new Node(root);
        rootNode.left = build(postOrder, inOrder, postStart, postStart + leftSize - 1, inStart, inIndex - 1);
        rootNode.right = build(postOrder, inOrder, postEnd - rightSize + 1, postEnd, inIndex + 1, inEnd);
        return rootNode;
    }

    public static void main(String[] args) {
        Node root = bulidTree();
        LinkedList<Node> queue = new LinkedList<>();
        queue.add(root);
        while (!queue.isEmpty()) {
            Node poll = queue.poll();
            if (poll.left != null) {
                queue.add(poll.left);
            }
            if (poll.right != null) {
                queue.add(poll.right);
            }
            System.out.printf("%d%s", poll.value, queue.isEmpty() ? "
" : " ");
        }
    }
}