BOJ - 1708 - 볼록 껍질

import sys

def getDegree(p1, p2):
    return p2[0] - p1[0], p2[1] - p1[1]

def ccw(p1, p2, p3):
    v, u = getDegree(p1, p2), getDegree(p2, p3)
    if v[0] * u[1] > v[1] * u[0]: return True
    else: return False

def convex_hull(position):
    convex = []
    for p3 in position:
        while len(convex) >= 2:
            p1, p2 = convex[-2], convex[-1]
            if ccw(p1, p2, p3):
                break
            convex.pop()
        convex.append(p3)
    return len(convex)

def solution():
    N = int(input())
    ret = 0
    position = []
    for _ in range(N):
        position.append(list(map(int, sys.stdin.readline().split())))

    position = sorted(position, key=lambda x:(x[0], x[1]))
    ret += convex_hull(position)

    position.reverse()
    ret += convex_hull(position)
    print(ret - 2)

solution()

https://www.acmicpc.net/problem/1708