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You are given n integers a1, a2, ..., an. Find the number of pairs of indexes i, j (i < j) that ai + aj is a power of 2 (i. e. some integer x exists so that ai + aj = 2x).
The first line contains the single positive integer n (1 ≤ n ≤ 105) — the number of integers.
The second line contains n positive integers a1, a2, ..., an (1 ≤ ai ≤ 109).
Print the number of pairs of indexes i, j (i < j) that ai + aj is a power of 2.
4
7 3 2 1
2
3
1 1 1
3
第一行说明输入多少个数
第二行n个数
求里面有多少组数的和为2的k次方
#include <bits/stdc++.h>
using namespace std;
#define ll long long
map<int,int>M;
int main()
{
ll n;
cin >> n;
ll ans = 0;
ll a;
ll Fuc = 1;
while(n--)
{
cin >> a;
for(int i = 0; i < 32; i++)
{
if (Fuc < a)
{
Fuc *= 2;
continue;
}
else
{
ans += M[Fuc-a];
Fuc *= 2;
}
}
Fuc = 1;
M[a]++;
}
cout << ans << endl;
}
//小心超时哦
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原文地址：http://www.cnblogs.com/yakoazz/p/5721331.html
