**Pair Sums to Power of Two - Google Top Interview Questions**

### Problem Statement :

You are given a list of integers nums. Return the number of pairs i < j such that nums[i] + nums[j] is equal to 2 ** k for some 0 ≤ k. Constraints 0 ≤ n ≤ 100,000 where n is the length of nums Example 1 Input nums = [1, 1, 3, 5] Output 4 Explanation We can have the following pairs that sums to power of 2 (1, 1) (1, 3) (1, 3) (3, 5)

### Solution :

` ````
Solution in C++ :
int solve(vector<int>& arr) {
unordered_map<int, int> mp;
for (int i : arr) mp[i]++;
int n = arr.size();
int ans = 0;
for (int i = 0; i < 32; i++) {
int sum = 1 << i;
for (int i = 0; i < n; i++) {
ans += mp[sum - arr[i]];
if (sum - arr[i] == arr[i]) ans--;
}
}
return ans / 2;
}
```

` ````
Solution in Java :
import java.util.*;
class Solution {
public int solve(int[] nums) {
ArrayList<Integer> powers = new ArrayList();
long two = 1;
while (two < Integer.MAX_VALUE) {
powers.add((int) (two));
two *= 2;
}
HashMap<Integer, Integer> hm = new HashMap();
int n = nums.length;
int res = 0;
for (int i = 0; i < n; i++) {
int cur = nums[i];
for (int twos : powers) {
res += hm.getOrDefault(twos - cur, 0);
}
hm.put(cur, hm.getOrDefault(cur, 0) + 1);
}
return res;
}
}
```

` ````
Solution in Python :
class Solution:
def solve(self, nums):
found = {}
ans = 0
powers = list(map(lambda x: 2 ** x, range(32)))
for num in nums:
for p in powers:
target = p - num
ans += found.get(target, 0)
found[num] = found.get(num, 0) + 1
return ans
```

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