力扣hot100 - 199、二叉树的右视图

题目：思路：由二叉树的层序遍历，可以一层一层的遍历每一层元素，当遍历到每一层最后一个加入结果数组即可。层序遍历可以看我上期文章。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<Integer> rightSideView(TreeNode root) {
        List<Integer> res = new ArrayList();
        Queue<TreeNode> q = new LinkedList();
        if(root == null) return res;
        q.add(root);

        while(!q.isEmpty()){
            int l = q.size();
            while(l > 0){
            TreeNode cur = q.peek();
            q.poll();
            if(l == 1){
                res.add(cur.val);
            }
            l--;
            if(cur.left != null){
                q.add(cur.left);
            }
            if(cur.right != null){
                q.add(cur.right);
            }
            }
        }
        return res;
    }
}