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【题目】

B - Powers of two

Time limit : 2sec / Memory limit : 1024MB

Score : 600 points

Problem Statement

Takahashi has N balls with positive integers written on them. The integer written on the i-th ball is Ai. He would like to form some number of pairs such that the sum of the integers written on each pair of balls is a power of 2. Note that a ball cannot belong to multiple pairs. Find the maximum possible number of pairs that can be formed.

Here, a positive integer is said to be a power of 2 when it can be written as 2t using some non-negative integer t.

Constraints

• 1≤N≤2×105
• 1≤Ai≤109
• Ai is an integer.

Input

Input is given from Standard Input in the following format:

NA1 A2 … AN

Output

Print the maximum possible number of pairs such that the sum of the integers written on each pair of balls is a power of 2.

Sample Input 1

31 2 3

Sample Output 1

1

We can form one pair whose sum of the written numbers is 4 by pairing the first and third balls. Note that we cannot pair the second ball with itself.

Sample Input 2

53 11 14 5 13

Sample Output 2

2

【题解】

题意：给定长度为n的序列，输出最多的两两组合之和为2的整次方的对数。

按理说我当时也是从后往前匹配的思路啊，但是为什么wa了...谜（留个悬念）

做法就是存入序列multiset自动排序，然后从大到小寻找是否存在可匹配满足条件的项，如果存在，答案+1，并删去匹配项。

认识了新容器 --- multiset，加深了迭代器的印象（太久没用都忘了）。

【代码】

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