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题目

• 原题连接：199. 二叉树的右视图

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1- 思路

• 使用二叉树的层序遍历

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2- 实现

⭐199. 二叉树的右视图——题解思路

class Solution {
    public List<Integer> rightSideView(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        if(root == null){
            return res;
        }
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while(!queue.isEmpty()){
            int len = queue.size();
            List<Integer> iterm = new ArrayList<>();
            while(len>0){
                TreeNode nowNode = queue.poll();
                iterm.add(nowNode.val);
                if(nowNode.left!=null){
                    queue.offer(nowNode.left);
                }
                if(nowNode.right!=null){
                    queue.offer(nowNode.right);
                }
                len--;
            }
            res.add(iterm.get(iterm.size()-1));
        }
        return res;
    }
}

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3- ACM 实现

public class leftView {

    static class TreeNode{
        int val;
        TreeNode left;
        TreeNode right;
        TreeNode(){}
        TreeNode(int x){
            val = x;
        }
    }

    public static TreeNode build(Integer[] nums){
        // 借助 queue
        Queue<TreeNode> queue = new LinkedList<>();
        TreeNode root = new TreeNode(nums[0]);
        queue.offer(root);
        int index = 1;
        while (!queue.isEmpty()){
            TreeNode nowNode = queue.poll();
            if(index<nums.length && nums[index]!=null){
                nowNode.left = new TreeNode(nums[index]);
                queue.offer(nowNode.left);
            }
            index++;
            if(index<nums.length && nums[index]!=null){
                nowNode.right = new TreeNode(nums[index]);
                queue.offer(nowNode.right);
            }
            index++;
        }
        return root;
    }

    public static List<Integer> leftView(TreeNode root){
        List<Integer> res = new ArrayList<>();
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while (!queue.isEmpty()){
            int len = queue.size();
            List<Integer> iterm = new ArrayList<>();
            while(len>0){
                TreeNode nowNode = queue.poll();
                iterm.add(nowNode.val);
                if(nowNode.left!=null){
                    queue.offer(nowNode.left);
                }
                if(nowNode.right!=null){
                    queue.offer(nowNode.right);
                }
                len--;
            }
            res.add(iterm.get(iterm.size()-1));
        }
        return res;
    }

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        String input = sc.next();
        input = input.replace("[","");
        input = input.replace("]","");
        String[] parts = input.split(",");
        Integer[] nums = new Integer[parts.length];
        for(int i = 0 ; i < parts.length;i++){
            if(parts[i].equals("null")){
                nums[i] = null;
            }else{
                nums[i] = Integer.parseInt(parts[i]);
            }
        }

        TreeNode root = build(nums);
        List<Integer> forRes = leftView(root);
        System.out.println(forRes.toString());
    }
}

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