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.

Example:

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

   1            <---
 /   \
2     3         <---
 \     \
  5     4       <---

 

Approach #1: C++. [recursive]

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> rightSideView(TreeNode* root) {
        vector<int> ans;
        helper(root, ans, 1);
        return ans;
    }
private:
    void helper(TreeNode* root, vector<int>& ans, int level) {
        if (root == NULL) return;
        if (ans.size() < level) ans.push_back(root->val);
        helper(root->right, ans, level+1);
        helper(root->left, ans, level+1);
    }
};

　　

Approach #2: Java. [bfs + queue]

/**
 * 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> res = new ArrayList();
        Queue<TreeNode> queue = new LinkedList();
        if (root == null) return res;
        queue.offer(root);
        while (!queue.isEmpty()) {
            int size = queue.size();
            for (int i = 0; i < size; ++i) {
                TreeNode cur = queue.poll();
                if (i == 0) res.add(cur.val);
                if (cur.right != null) queue.offer(cur.right);
                if (cur.left != null) queue.offer(cur.left);
            }
        }
        return res;
    }
}

　　

Approach #3: Python. [Iterator]

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def rightSideView(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        view = []
        if root:
            level = [root]
            while level:
                view += level[-1].val,
                level = [kid for node in level for kid in (node.left, node.right) if kid]
                
        return view