Codeforces Round #383 (Div. 2) B. Arpa’s obvious problem and Mehrdad’s terrible solution

题目链接 ：http://codeforces.com/contest/742/problem/B

题意：给你n个数和一个x，问你n个数中有几对(ai , aj)的xor值为x，而且j>i。

题目比较简单看一下代码就会秒懂了。还有要注意结果用longlong来存。

#include <iostream>
#include <cstring>
#include <algorithm>
#include <cstdio>
using namespace std;
const int M = 1e5 + 10;
const int mmax = 3e6;
int a[M] , b[mmax];
int main() {
    int n , x;
    scanf("%d%d" , &n , &x);
    for(int i = 1 ; i <= n ; i++) {
        scanf("%d" , &a[i]);
        b[a[i]]++;
    }
    long long count = 0;
    for(int i = 1 ; i <= n ; i++) {
        b[a[i]]--;
        if(b[x ^ a[i]]) {
            count += b[x ^ a[i]];
        }
    }
    printf("%lld
" , count);
    return 0;
}