[LeetCode][199. Binary Tree Right Side View] 2 Approaches: DFS and BFS with Detailed Explanation

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By Long Luo

This article is the solution 2 Approaches: DFS and BFS with Detailed Explanation of Problem 199. Binary Tree Right Side View.

Here shows 2 Approaches to slove this problem: DFS and BFS.

DFS

We traverse the tree in the order of root node→right subtree→left subtree\textit{root node} \to \textit{right subtree} \to \textit{left subtree} to ensure that each level traverse the rightmost node first.

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public List<Integer> rightSideView_dfs(TreeNode root) {
    List<Integer> ans = new ArrayList<>();
    if (root == null) {
        return ans;
    }

    dfs(root, 1, ans);
    return ans;
}

public void dfs(TreeNode root, int depth, List<Integer> numList) {
    if (root == null) {
        return;
    }

    if (numList.size() < depth) {
        numList.add(root.val);
    }

    dfs(root.right, depth + 1, numList);
    dfs(root.left, depth + 1, numList);
}

Analysis

  • Time Complexity: O(n)O(n)
  • Space Complexity: O(n)O(n)

BFS

We can use BFS to traverse the levels of the tree and record the last element of each level.

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public List<Integer> rightSideView_bfs(TreeNode root) {
    List<Integer> ans = new ArrayList<>();
    if (root == null) {
        return ans;
    }

    Queue<TreeNode> queue = new LinkedList<>();
    queue.offer(root);
    while (!queue.isEmpty()) {
        int size = queue.size();
        for (int i = size - 1; i >= 0; i--) {
            TreeNode node = queue.poll();
            if (i == size - 1) {
                ans.add(node.val);
            }

            if (node.right != null) {
                queue.offer(node.right);
            }

            if (node.left != null) {
                queue.offer(node.left);
            }
        }
    }

    return ans;
}

Analysis

  • Time Complexity: O(n)O(n).
  • Space Complexity: O(n)O(n).

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