LeetCode link

first thought

  • DFS.

solution1

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public class Solution {
    public List<Integer> rightSideView(TreeNode root) {
        ArrayList<Integer> result = new ArrayList<>();
        this.rightSideView(root, 0, result);
        return result;
    }
    private void rightSideView(TreeNode node, int level, ArrayList<Integer> result) {
        if (node == null) {
            return;
        }
        if (result.size() <= level) {
            result.add(node.val);
            this.rightSideView(node.left, level + 1, result);
            this.rightSideView(node.right, level + 1, result);
        }
        else {
            result.set(level, node.val);
            this.rightSideView(node.left, level + 1, result);
            this.rightSideView(node.right, level + 1, result);
        }
    }
}

solution2

BFS

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class Solution {
    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()) {
            Queue<TreeNode> queue2 = new LinkedList<>();
            TreeNode lastNode = queue.peek();
            while (!queue.isEmpty()) {
                lastNode = queue.poll();
                if (lastNode.left != null) {
                    queue2.add(lastNode.left);
                }
                if (lastNode.right != null) {
                    queue2.add(lastNode.right);
                }
            }
            result.add(lastNode.val);
            queue = queue2;
        }
        return result;
    }
}