### outerTrees 
from typing import *
def f_gold(trees: List[List[int]]) -> List[List[int]]:
    def cross(i, j, k):
        a, b, c = trees[i], trees[j], trees[k]
        return (b[0] - a[0]) * (c[1] - b[1]) - (b[1] - a[1]) * (c[0] - b[0])
    n = len(trees)
    if n < 4:
        return trees
    trees.sort()
    vis = [False] * n
    stk = [0]
    for i in range(1, n):
        while len(stk) > 1 and cross(stk[-2], stk[-1], i) < 0:
            vis[stk.pop()] = False
        vis[i] = True
        stk.append(i)
    m = len(stk)
    for i in range(n - 2, -1, -1):
        if vis[i]:
            continue
        while len(stk) > m and cross(stk[-2], stk[-1], i) < 0:
            stk.pop()
        stk.append(i)
    stk.pop()
    return [trees[i] for i in stk]