打印二叉树的左视图或者右视图

给定一个二叉树，打印从左边看到的，或者从右边看到的

[力扣链接](https://leetcode-cn.com/problems/binary-tree-right-side-view/](https://leetcode-cn.com/problems/binary-tree-right-side-view/)

分析：可以参考广度优先遍历
解法1

public List<Integer> rightSideView(TreeNode root) {
        Queue<TreeNode> queue = new LinkedList<>();
        List<Integer> result = new ArrayList<>();
        queue.offer(root);
        while(!queue.isEmpty()) {
            int size = queue.size();
               //对每一行操作
            while(size > 0) {
                
                TreeNode current = queue.poll();
                
                if (current.left!=null) {
                    queue.offer(current.left);
                }
                if (current.right != null) {
                    queue.offer(current.right);
                }
                
                size --;
                //表示是每一行的最后一个
                if (size == 0) {
                    result.add(current.value);
                }
                
            }
        }
        return result;
    }

解法2 递归

public List<Integer> rightSideView(TreeNode root) {
        List<Integer> list = new ArrayList<>();
       rightSideView(root,list,0);
        return list;
    }
     public void rightSideView(TreeNode root, List<Integer> list ,int level) {
        if (root == null) {
            return;
        }
        
        if (level == list.size()) {
            list.add(root.val);
        }
        rightSideView(root.right,list,level+1);
        rightSideView(root.left,list,level + 1);
        
    }

打印左视图和右视图相似，只需在加入队列的时候先加入左结点