백준 1940번

c++

#include <iostream>
#include <algorithm>

int n,m = 0;
int arr[15001] = {0,};
int cnt = 0;

int main(){
    std::cin >> n >> m;
    for (int i = 0;i < n;i++){
        std::cin >> arr[i];
    }
    std::sort(arr,arr+n);

    for (int i = 0,j = n - 1;i < j;){
        int res = arr[i]+arr[j];
        if (res==m){
            cnt += 1;
            i++;
            j--;
        }
        else if (arr[i] < m - arr[j]){
            i++;
        }
        else if (arr[j] > m - arr[i]){
            j--;
        }
    }

    std::cout << cnt;
    return 0;
}

python

import math

n = int(input(''))
m = int(input(''))

arr = list(map(int,input('').split(' ')))
arr.sort()
cnt = 0
last = math.inf


for i in range(len(arr)):
    if i == last:
        break
    start = arr[i]
    if (m-start) in arr and arr.index(m-start) > i:
        last = m-start
        cnt += 1
        continue


print(cnt,end='')