199. Binary Tree Right Side View

2020, Nov 18 One min read

1. Description

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.

2. Example

Example 1

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

Example 2

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

Example 3

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

Example 4

Input: root = []
Output: []

3. Constraints

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

4. Solutions

Breadth-first Search

n is the number of nodes in root
Time complexity: O(n)
Space complexity: O(n)

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class Solution {
public:
    vector<int> rightSideView(TreeNode *root) {
        vector<int> right_view_values;
        if (root == nullptr) {
            return right_view_values;
        }

        queue<TreeNode *> level_nodes{{root}};
        while (!level_nodes.empty()) {
            const int n = level_nodes.size();

            int value = 0;
            for (int i = 0; i < n; ++i) {
                TreeNode *node = level_nodes.front();
                level_nodes.pop();

                value = node->val;
                if (node->left != nullptr) {
                    level_nodes.push(node->left);
                }
                if (node->right != nullptr) {
                    level_nodes.push(node->right);
                }
            }

            right_view_values.push_back(value);
        }

        return right_view_values;
    }
};