题解 | #输出二叉树的右视图# 先重建，再层序遍历

import java.util.*;


class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;
    public TreeNode(int val) {
        this.val = val;
    }
}

public class Solution {
    private Map<Integer, Integer> inMap = new HashMap<>();
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 求二叉树的右视图
     * @param preOrder int整型一维数组 先序遍历
     * @param inOrder int整型一维数组 中序遍历
     * @return int整型一维数组
     */
    public int[] solve (int[] preOrder, int[] inOrder) {
        // write code here
        for (int i = 0; i < inOrder.length; i++) {
            inMap.put(inOrder[i], i);
        }
        TreeNode root = buildTree(preOrder, inOrder, 0 , preOrder.length - 1, 0, inOrder.length - 1);
        Deque<TreeNode> que = new LinkedList<>();
        if (root != null) {
            que.offer(root);
        }
        List<Integer> list = new ArrayList<>();
        while (!que.isEmpty()) {
            list.add(que.peekLast().val);
            int leng = que.size();
            for (int i = 0; i < leng; i++) {
                TreeNode current = que.poll();
                if (current.left != null) {
                    que.offer(current.left);
                }
                if (current.right != null) {
                    que.offer(current.right);
                }
            }
        }
        int[] ans = new int[list.size()];
        for (int i = 0; i < ans.length; i++) {
            ans[i] = list.get(i);
        }
        return ans;
    }

    private TreeNode buildTree(int[] preOrder, int[] vinOrder, int preStart,
                               int preEnd, int inLeft, int inEnd) {
        if (preEnd < preStart) {
            return null;
        }
        int current = preOrder[preStart];
        TreeNode root = new TreeNode(current);
        int currentIndex = inMap.get(current);
        int leftLeng = currentIndex - inLeft;
        root.left = buildTree(preOrder, vinOrder, preStart + 1, preStart + leftLeng,
                              inLeft, currentIndex - 1);
        root.right = buildTree(preOrder, vinOrder, preStart + leftLeng  + 1, preEnd,
                               currentIndex + 1, inEnd);
        return root;
    }
}

先重建二叉树，再层序遍历