39. 199. Binary Tree Right Side View

/**
 * 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> ans = new ArrayList<Integer>();
        if(root==null){
            return ans;
        }
       
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        queue.add(root);
        while(!queue.isEmpty()){
            int levelSize = queue.size();
            for(int i=0;i<levelSize;i++){
                TreeNode node = queue.peek();
                queue.poll();
                if(i==(levelSize-1)){
                    ans.add(node.val);
                }

                if(node.left!=null){
                    queue.add(node.left);
                }
                if(node.right!=null){
                    queue.add(node.right);
                }
            }
           
        }
         return ans;
    }
}