BOJ - 2254 - 감옥 건설

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(p1, 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 convex

def isInside(X, Y, position):
    cross = 0
    for i in range(len(position)):
        j = (i + 1) % len(position)
        if (position[i][1] > Y) != (position[j][1] > Y):
            atx = (position[j][0] - position[i][0]) * (Y - position[i][1]) / (position[j][1] - position[i][1]) + position[i][0]
            if X < atx:
                cross += 1
    return True if cross % 2 > 0 else False

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

    ret = 0
    while True:
        if len(position) < 3: break

        position = sorted(position, key=lambda x:(x[0], x[1]))
        ccwtemp = convex_hull(position)
        position.reverse()
        temp = convex_hull(position)

        for i in range(1, len(temp)-1):
            ccwtemp.append(temp[i])

        if len(ccwtemp) < 3: break

        if not isInside(X, Y, ccwtemp):
            ret -= 1

        tempdata = []
        for i in range(len(position)):
            if position[i] not in ccwtemp:
                tempdata.append(position[i])
        position = tempdata
        del tempdata

        ret += 1

    print(ret)
solution()

https://www.acmicpc.net/problem/2254