题解 | #输出二叉树的右视图#

import java.util.*;
public class Solution {
    public int[] solve (int[] preOrder, int[] inOrder) {
        //层次遍历输出最右边元素
        if(preOrder.length <= 1) return preOrder;
        Queue<TreeNode> queue = new LinkedList<>();
        List<Integer> list = new LinkedList<>();
        queue.add(creat(preOrder, 0, preOrder.length - 1, inOrder, 0, inOrder.length - 1));
        while(!queue.isEmpty()){
            int size = queue.size();//记录这一层有多少个节点
            for(int i = 0; i < size; i ++){
                TreeNode node = queue.poll();
                if(i == size - 1) {//最后一个
                    list.add(node.val);
                }
                if(node.left != null) queue.add(node.left);
                if(node.right != null) queue.add(node.right);
            }
        }
        int[] result = new int[list.size()];
        for(int i = 0; i < result.length; i++){
            result[i] = list.get(i);
        }
        return result;
    }

    TreeNode creat(int[] pre, int leftPre, int rightPre, int[] in, int leftIn, int rightIn){
        if(leftPre > rightPre || leftIn > rightIn) return null;
        TreeNode root = new TreeNode(pre[leftPre]);

        for(int i = leftIn; i <= rightIn; i ++){
            if(in[i] == pre[leftPre]){
                root.left = creat(pre, leftPre + 1, leftPre + i - leftIn, in, leftIn, i - 1);
                root.right = creat(pre, i - leftIn + leftPre + 1, rightPre, in, i + 1, rightIn);
                break;
            }
        }
        return root;
    }
}