Leetcode #1712: Ways to Split Array Into Three Subarrays
In this guide, we solve Leetcode #1712 Ways to Split Array Into Three Subarrays in Python and focus on the core idea that makes the solution efficient.
You will see the intuition, the step-by-step method, and a clean Python implementation you can use in interviews.
Problem Statement
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.
Quick Facts
- Difficulty: Medium
- Premium: No
- Tags: Array, Two Pointers, Binary Search, Prefix Sum
Intuition
The constraints hint that we can reason about two ends of the data at once, which is perfect for a two-pointer scan.
Moving one pointer at a time keeps the invariant intact and avoids nested loops.
Approach
Place pointers at the left and right ends and move them based on the comparison or target condition.
This yields a clean linear pass after any required sorting.
Steps:
- Set left and right pointers.
- Move a pointer based on the condition.
- Update the best answer while scanning.
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
The time complexity is , where is the length of the array . The space complexity is O(1).
Edge Cases and Pitfalls
Watch for boundary values, empty inputs, and duplicate values where applicable. If the problem involves ordering or constraints, confirm the invariant is preserved at every step.
Summary
This Python solution focuses on the essential structure of the problem and keeps the implementation interview-friendly while meeting the constraints.