199. Binary Tree Right Side View

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].

Tree类题目。

注意到每层有且仅有一个值，而且是每一层最靠近右边的。 可以采用DFS的思路，每次先traverse右孩子。如果level等于res的size，则把该值存入res，否则不是该层最靠右的。

/**
 * 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> res = new ArrayList<Integer>();
        helper(root, 0, res);
        return res;
    }
    
    private void helper(TreeNode node, int level, List<Integer> res) {
        if(node == null) return;
        if(level == res.size()) {
            res.add(node.val);
        }
        helper(node.right, level+1, res);
        helper(node.left, level+1, res);
    }
}