Ways to Split Array Into Three Subarrays — LeetCode 1712 Python Solution

The problem

A split of an integer array is good if: The array is split into three non-empty contiguous subarrays - named left, mid, right respectively from left to right. The sum of the elements in left is less than or equal to the sum of the elements in mid, and the sum of the elements in mid is less than or equal to the sum of the elements in right.

Example

Input

nums = [1,1,1]

Output

1

Explanation

The only good way to split nums is [1] [1] [1].

Python solution

class Solution:
    def waysToSplit(self, nums: List[int]) -> int:
        mod = 10**9 + 7
        s = list(accumulate(nums))
        ans, n = 0, len(nums)
        for i in range(n - 2):
            j = bisect_left(s, s[i] << 1, i + 1, n - 1)
            k = bisect_right(s, (s[-1] + s[i]) >> 1, j, n - 1)
            ans += k - j
        return ans % mod

Complexity

Pattern: Prefix Sum

Precompute running totals once so any range query becomes a single subtraction. LeetCode 1712. Ways to Split Array Into Three Subarrays is filed here because LeetCode tags it Prefix Sum, which is the vocabulary this hub collects.

The prefix sum guide has the Python template for the pattern and the 157 LeetCode problems that use it.

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