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
class Solution {
long mod = 1_000_000_007;
public int numOfSubarrays(int[] arr) {
// Map of odd sum and even sum -> frequency
HashMap<Integer, Integer> map = new HashMap<>();
map.put(0, 1);
map.put(1, 0);
long count = 0;
long sum = 0;
for (int x : arr) {
sum += x;
if (sum % 2 == 0) {
count = (count + map.get(1)) % mod;
map.put(0, map.get(0) + 1);
} else {
count = (count + map.get(0)) % mod;
map.put(1, map.get(1) + 1);
}
}
return (int) count;
}
}