1、宽度优先遍历

public List largestValues(TreeNode root) {
    if (root == null) {
        return new ArrayList<>();
    }
    List res = new ArrayList<>();
    Queue queue = new ArrayDeque<>();
    queue.offer(root);
    while (!queue.isEmpty()) {
        int len = queue.size();
        int maxVal = Integer.MIN_VALUE;
        while (len > 0) {
            len--;
            TreeNode t = queue.poll();
            maxVal = Math.max(maxVal, t.val);
            if (t.left != null) {
                queue.offer(t.left);
            }
            if (t.right != null) {
                queue.offer(t.right);
            }
        }
        res.add(maxVal);
    }
    return res;
}

2、递归

public List largestValues(TreeNode root) {
    List list = new ArrayList<>();
    dfs(root,0,list);
    return list;
}

private void dfs(TreeNode root, int height,List list) {
    if (root == null){
        return;
    }
    if (list.size() <= height){
        list.add(height,root.val);
    }else {
        list.set(height,Math.max(list.get(height),root.val));
    }
    height++;
    dfs(root.left,height,list);
    dfs(root.right,height,list);
}