LeetCode link

first thought

  • DFS

DFS solution

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

BFS solution

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class Solution {
    public List<Integer> largestValues(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        if (root == null) {
            return result;
        }
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        while (!queue.isEmpty()) {
            int max = Integer.MIN_VALUE;
            Queue<TreeNode> queue2 = new LinkedList<>();
            while (!queue.isEmpty()) {
                TreeNode node = queue.poll();
                max = Math.max(max, node.val);
                if (node.left != null) {
                    queue2.add(node.left);
                }
                if (node.right != null) {
                    queue2.add(node.right);
                }
            }
            result.add(max);
            queue = queue2;
        }
        return result;
    }
}