1、题目

2、分析与解答

class Solution {
    public int countPairs(int[] deliciousness) {
        final int mod = 1000000007;
        int max = 0;
        for(int deli : deliciousness) {
            max = Math.max(deli,max);
        }
        int maxValue = 2 * max;
        int pairs = 0;
        Map<Integer,Integer> map = new HashMap<>();
        for(int i = 0;i < deliciousness.length;i++) {
            int val = deliciousness[i];
            for(int sum = 1;sum <= maxValue; sum<<=1) {
                int count = map.getOrDefault(sum - val , 0);
                pairs = (pairs + count) % mod;
            }
            map.put(val,map.getOrDefault(val,0) + 1);
        }
        return pairs;
    }
}