Example 1:

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

Example 2:

Input: root = [1,null,3]
Output: [1,3]

Example 3:

Input: root = []
Output: []

Constraints:

• The number of nodes in the tree is in the range [0, 100].
• -100 <= Node.val <= 100

Solution: Pre-order traversal

By using pre-order traversal, the right most node will be the last one to visit in each level.

Time complexity: O(n)
Space complexity: O(|height|)

// Author: Huahua
class Solution {
public:
  vector<int> rightSideView(TreeNode* root) {
    vector<int> ans;
    function<void(TreeNode*, int)> dfs = [&](TreeNode* root, int d) {
      if (!root) return;
      if (ans.size() < d + 1) ans.push_back(0);
      ans[d] = root->val;
      dfs(root->left, d + 1);
      dfs(root->right, d + 1);
    };
    dfs(root, 0);
    return ans;
  }
};

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