199. Binary Tree Right Side View

/*
199. Binary Tree Right Side View 
Total Accepted: 38607 Total Submissions: 113726 Difficulty: Medium
Given a binary tree, imagine yourself standing on the right side of it, return the values of the nodes you can see ordered from top to bottom.

For example:
Given the following binary tree,
   1            <---
 /   \
2     3         <---
 \     \
  5     4       <---
You should return [1, 3, 4].

Credits:
Special thanks to @amrsaqr for adding this problem and creating all test cases.

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*/


/**
 * Definition for a binary tree node.
 */
 
import java.util.*;

 class TreeNode {
     int val;
     TreeNode left;
     TreeNode right;
     TreeNode(int x) { val = x; }
 }
 
 public class RightSideView_199 {
     public static void main(String[] args) {
         TreeNode root = new TreeNode(1);
         TreeNode two = new TreeNode(2);
         TreeNode three = new TreeNode(3);
         root.left = two;
         root.right = three;
         
         TreeNode four = new TreeNode(4);
         TreeNode five = new TreeNode(5);
         
         two.right = five;
        // three.right = four;
         
         System.out.print(rightSideView(root));
     }
     
     
    public static List rightSideView(TreeNode root) {
         //https://leetcode.com/discuss/30464/reverse-level-order-traversal-java
         // Reverse Level Order Traversal, java
         List list = new ArrayList<>();
         Queue queue = new LinkedList<>();
         if (root == null) return list;
         
         queue.offer(root);
         
         while(queue.size() != 0) {
             int size = queue.size();
             for(int i=0; i < size; i++) {
                 TreeNode cur = queue.poll();
                 if (i == 0) list.add(cur.val);
                 if (cur.right != null) queue.offer(cur.right);
                 if (cur.left != null) queue.offer(cur.left);
             }
         }
         return list;
    } 
     
    public static List rightSideView_sol2(TreeNode root) {
        /* The core idea of this algorithm:
        1. Each depth of the tree only select one node
        2. View depth is current size of result list
            https://leetcode.com/discuss/31348/my-simple-accepted-solution-java
            */
        List list = new ArrayList<>();
        helper(list, root, 0);
        return list;
    }
    
    public static void helper(List list, TreeNode root, int depth) {
        if (root == null) return; 
        if (depth == list.size()) {
            list.add(root.val);
        }
        helper(list, root.right, depth + 1);
        helper(list, root.left, depth + 1);
    }
}