199. Binary Tree Right Side View

Imagine standing on the right side of a binary tree. Return the values of the nodes you can see, from top to bottom.

Traverse level by level. The last node of each level is the one you see from the right.

1function rightSideView(root: TreeNode | null): number[] {
2  const res: number[] = [];
3  let level = root ? [root] : [];
4  while (level.length) {
5    res.push(level.at(-1)!.val);
6    level = level.flatMap((n) => [n.left, n.right].filter((c): c is TreeNode => !!c));
7  }
8  return res;
9}

tree

level

view

empty

First level is the root.

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