157. Binary Tree Right Side View

Perform BFS level by level. The last node at each level is visible from the right side.

Approach

1. BFS with a queue, processing level by level.
2. At each level, the last node dequeued is the rightmost.
3. Append it to the result.

Complexity

• Time: O(n)
• Space: O(n)

import java.util.*;

class Solution {
    static class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;
        TreeNode(int val) { this.val = val; }
    }

    public List<Integer> rightSideView(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        if (root == null) return result;

        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);

        while (!queue.isEmpty()) {
            int levelSize = queue.size();
            for (int i = 0; i < levelSize; i++) {
                TreeNode node = queue.poll();
                if (i == levelSize - 1) result.add(node.val);
                if (node.left != null) queue.add(node.left);
                if (node.right != null) queue.add(node.right);
            }
        }

        return result;
    }
}