1524. Number of Sub-arrays With Odd Sum -----------M

 Given an array of integers arr. Return the number of sub-arrays with odd sum.

As the answer may grow large, the answer must be computed modulo 10^9 + 7.

 

Example 1:

Input: arr = [1,3,5]
Output: 4
Explanation: All sub-arrays are [[1],[1,3],[1,3,5],[3],[3,5],[5]]
All sub-arrays sum are [1,4,9,3,8,5].
Odd sums are [1,9,3,5] so the answer is 4.

Example 2:

Input: arr = [2,4,6]
Output: 0
Explanation: All sub-arrays are [[2],[2,4],[2,4,6],[4],[4,6],[6]]
All sub-arrays sum are [2,6,12,4,10,6].
All sub-arrays have even sum and the answer is 0.

Example 3:

Input: arr = [1,2,3,4,5,6,7]
Output: 16

Example 4:

Input: arr = [100,100,99,99]
Output: 4

Example 5:

Input: arr = [7]
Output: 1

 

Constraints:

• 1 <= arr.length <= 10^5
• 1 <= arr[i] <= 100

A:

class Solution {
public:
    int numOfSubarrays(vector<int>& arr) {
        int n = arr.size();
        if(n==0)
            return 0;
        
        for(int i =0;i<n;i++){
            arr[i] = (arr[i]  + (i==0?0:arr[i-1]))%2;// %2 can be resplaced with &1
        }
        int countOf1 = arr[0];
        int res = arr[0];
        for(int i =1; i<n;i++){
            int countOf0 = i - countOf1;
            if(arr[i]){
                res = (res + 1+countOf0) % 1000000007;
                countOf1++;
            }else{
                res += countOf1;
            }
        }
        return res;
    }
};

犯了个错误，是

忘记了update countOf1