题解 | #输出二叉树的右视图#

import java.util.*;


public class Solution {
    /**
     * 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 xianxu int整型一维数组 先序遍历
     * @param zhongxu int整型一维数组 中序遍历
     * @return int整型一维数组
     */
    public int[] solve (int[] xianxu, int[] zhongxu) {
        // write code here
        TreeNode root = reConstructBinaryTree(xianxu, zhongxu);
        List<Integer> list = fromRight(root);
        int[] ans = new int[list.size()];
        for (int i = 0; i < list.size(); i++) {
            ans[i] = list.get(i);
        }
        return ans;
    }
    public TreeNode reConstructBinaryTree(int[] pre, int[] vin) {
        if (pre.length == 0) {
            return null;
        }
        if (pre.length == 1) {
            return new TreeNode(pre[0]);
        }
        if (pre.length == 2) {
            TreeNode root = new TreeNode(pre[0]);
            if (pre[0] == vin[0]) {
                root.right = new TreeNode(vin[1]);
            } else {
                root.left = new TreeNode(vin[0]);
            }
            return root;
        }
        int rootVal = pre[0];
        int rootIndex = 0;
        for (int i = 0; i < vin.length; i++) {
            if (vin[i] == rootVal) {
                rootIndex = i;
                break;
            }
        }
        TreeNode root = new TreeNode(rootVal);
        if (rootIndex == 0) {
            int[] rightVin = Arrays.copyOfRange(vin, 1, vin.length);
            Map<Integer, Integer> map = new HashMap<>();
            for (int i : rightVin) {
                map.put(i, 0);
            }
            int endIndex = 0;
            for (int j = 0; j < pre.length; j++) {
                if (map.containsKey(pre[j])) {
                    endIndex = j;
                }
            }
            int[] rightPre = Arrays.copyOfRange(pre, 1, endIndex + 1);
            root.right = reConstructBinaryTree(rightPre, rightVin);
        } else if (rootIndex == 1) {
            root.left = new TreeNode(vin[0]);
            int[] rightVin = Arrays.copyOfRange(vin, 2, vin.length);
            Map<Integer, Integer> map = new HashMap<>();
            for (int i : rightVin) {
                map.put(i, 0);
            }
            int endIndex = 0;
            for (int j = 0; j < pre.length; j++) {
                if (map.containsKey(pre[j])) {
                    endIndex = j;
                }
            }
            int[] rightPre = Arrays.copyOfRange(pre, 2, endIndex + 1);
            root.right = reConstructBinaryTree(rightPre, rightVin);
        } else if (rootIndex == vin.length - 1) {
            int[] leftVin = Arrays.copyOfRange(vin, 0, vin.length - 1);
            Map<Integer, Integer> map = new HashMap<>();
            for (int i : leftVin) {
                map.put(i, 0);
            }
            int endIndex = 0;
            for (int j = 0; j < pre.length; j++) {
                if (map.containsKey(pre[j])) {
                    endIndex = j;
                }
            }
            int[] leftPre = Arrays.copyOfRange(pre, 1, endIndex + 1);
            root.left = reConstructBinaryTree(leftPre, leftVin);
        } else if (rootIndex == vin.length - 2) {
            root.right = new TreeNode(vin[vin.length - 1]);
            int[] leftVin = Arrays.copyOfRange(vin, 0, vin.length - 2);
            Map<Integer, Integer> map = new HashMap<>();
            for (int i : leftVin) {
                map.put(i, 0);
            }
            int endIndex = 0;
            for (int j = 0; j < pre.length; j++) {
                if (map.containsKey(pre[j])) {
                    endIndex = j;
                }
            }
            int[] leftPre = Arrays.copyOfRange(pre, 1, endIndex + 1);
            root.left = reConstructBinaryTree(leftPre, leftVin);
        } else {
            int[] leftVin = Arrays.copyOfRange(vin, 0, rootIndex);
            int[] rightVin = Arrays.copyOfRange(vin, rootIndex + 1, vin.length);
            Map<Integer, Integer> map = new HashMap<>();
            for (int i : leftVin) {
                map.put(i, 0);
            }
            int endIndex = 0;
            for (int j = 0; j < pre.length; j++) {
                if (map.containsKey(pre[j])) {
                    endIndex = j;
                }
            }
            int[] leftPre = Arrays.copyOfRange(pre, 1, endIndex + 1);
            int[] rightPre = Arrays.copyOfRange(pre, endIndex + 1, pre.length);
            root.left = reConstructBinaryTree(leftPre, leftVin);
            root.right = reConstructBinaryTree(rightPre, rightVin);
        }
        return root;
    }
    public List<Integer> fromRight(TreeNode root) {
        Map<Integer, Integer> map = new HashMap<>();
        Queue<TreeNode> treeQueue = new LinkedList<>();
        Queue<Integer> heightQueue = new LinkedList<>();
        int maxHeight = 0;
        treeQueue.add(root);
        heightQueue.add(maxHeight);
        while (!treeQueue.isEmpty()) {
            TreeNode node = treeQueue.poll();
            int height = heightQueue.poll();
            maxHeight = Math.max(maxHeight, height);
            if (!map.containsKey(height)) {
                map.put(height, node.val);
            }
            if (node.right != null) {
                treeQueue.add(node.right);
                heightQueue.add(height + 1);
            }
            if (node.left != null) {
                treeQueue.add(node.left);
                heightQueue.add(height + 1);
            }
        }
        List<Integer> ans = new ArrayList<>();
        for (int i = 0; i <= maxHeight; i++) {
            ans.add(map.get(i));
        }
        return ans;
    }
}