leetcode 199 Binary Tree Right Side View
z
 2019-04-07 00:38:57  

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:

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Input: [1,2,3,null,5,null,4]
Output: [1, 3, 4]
Explanation:

   1            <---
 /   \
2     3         <---
 \     \
  5     4       <---
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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<Integer> rightSideView(TreeNode root) {
        List<Integer> ans = new ArrayList<>();
        helper(ans, root, 0);
        return ans;
    }
    public void helper(List<Integer> ans, TreeNode root, int level) {
        if (root == null)
            return ;
        if (ans.size() == level) {
            ans.add(root.val);
        }   
        if (root.right != null)
            helper(ans, root.right, level+1);
        if (root.left != null)
            helper(ans, root.left, level+1);
    }
}