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

1524. Number of Sub-arrays With Odd Sum

https://leetcode.com/problems/number-of-sub-arrays-with-odd-sum/

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

代码

• 动态规划

class Solution:
    def numOfSubarrays(self, arr: List[int]) -> int:        
        dp = [0] * len(arr)
        dp[0] = arr[0] & 1
        
        for i in range(1, len(arr)):
            if arr[i] & 1 == 1:
                dp[i] = i - dp[i - 1] + 1
            else:
                dp[i] = dp[i - 1]
                
        return sum(dp) % (10**9 + 7)