[LC515]在每个树的行中找最大值

    public List<Integer> largestValues(TreeNode root) {
        Deque<TreeNode> queue = new ArrayDeque();
        if (root != null) queue.add(root);
        List<Integer> res = new ArrayList();
        while(!queue.isEmpty()){
            int sz = queue.size();
            Integer levelMax = Integer.MIN_VALUE;
            for(int i = 0; i < sz; i++){
                TreeNode node = queue.pollFirst();
                levelMax = Math.max(levelMax, node.val);
                if (node.left != null) queue.addLast(node.left);
                if (node.right != null) queue.addLast(node.right);
            }
            res.add(levelMax);
        }
        return res;
    }

    int maxDepth = 0;
    Map<Integer, Integer> map = new HashMap<>();
    public List<Integer> largestValues(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        dfs(root, 1);
        for (int i = 1; i <= max; i++) res.add(map.get(i));
        return res;
    }
    void dfs(TreeNode node, int depth) {
        if (node == null) return ;
        maxDepth = Math.max(maxDepth, depth);
        map.put(depth, Math.max(map.getOrDefault(depth, Integer.MIN_VALUE), node.val));
        dfs(node.left, depth + 1);
        dfs(node.right, depth + 1);
    }