3917. Count Indices With Opposite Parity

You are given an integer array nums of length n.

The score of an index i is defined as the number of indices j such that:

• i < j < n, and
• nums[i] and nums[j] have different parity (one is even and the other is odd).

Return an integer array answer of length n, where answer[i] is the score of index i.

class Solution {
public:
    vector<int> countOppositeParity(vector<int>& nums) {
        vector<int> res(nums.size());
        int parity[2]{0,};
        for(int i = nums.size() - 1; i >= 0; i--) {
            res[i] = parity[!(nums[i]&1)];
            parity[nums[i]&1]++;
        }
        return res;
    }
};