BinaryTreeRightSideView199.java

/**
 * 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.
 * 
 * Example:
 * 
 * Input: [1,2,3,null,5,null,4]
 * Output: [1, 3, 4]
 * 
 * Explanation:
 * 
 *    1            <---
 *  /   \
 * 2     3         <---
 *  \     \
 *   5     4       <---
 */

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */

public class BinaryTreeRightSideView199 {
    public List<Integer> rightSideView(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        rightSideView(root, res, 0);
        return res;
    }

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

}