Java solution of https://leetcode.com/problems/binary-tree-right-side-view/

leetcode_binary_tree_right_side_view_java_version.java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<Integer> rightSideView(TreeNode root) {
        List<Integer> arr = new ArrayList<Integer>();
        if(root == null){
            return arr;
        }
        List<TreeNode> oneLevel = new ArrayList<TreeNode>();
        List<TreeNode> nextLevel = new ArrayList<TreeNode>();
        oneLevel.add(root);
        arr.add(root.val);
        while(!oneLevel.isEmpty()){
            TreeNode current = oneLevel.remove(0) ;
            if(current.left != null){
                nextLevel.add(current.left);
            }
            
            if(current.right != null){
                nextLevel.add(current.right);
            }
            
            if(oneLevel.isEmpty()){
                if(!nextLevel.isEmpty()){
                    TreeNode last = nextLevel.get(nextLevel.size() - 1);
                    arr.add(last.val);
                    oneLevel = nextLevel;
                    nextLevel = new ArrayList<TreeNode>();
                }
            }
            
        }
        return arr;
    }
}