1013 Partition Array Into Three Parts With Equal Sum.py

#!/usr/bin/python3
"""
Given an array A of integers, return true if and only if we can partition the
array into three non-empty parts with equal sums.

Formally, we can partition the array if we can find indexes i+1 < j with (A[0]
+ A[1] + ... + A[i] == A[i+1] + A[i+2] + ... + A[j-1] == A[j] + A[j-1] + ... +
A[A.length - 1])

Example 1:

Input: [0,2,1,-6,6,-7,9,1,2,0,1]
Output: true
Explanation: 0 + 2 + 1 = -6 + 6 - 7 + 9 + 1 = 2 + 0 + 1
Example 2:

Input: [0,2,1,-6,6,7,9,-1,2,0,1]
Output: false
Example 3:

Input: [3,3,6,5,-2,2,5,1,-9,4]
Output: true
Explanation: 3 + 3 = 6 = 5 - 2 + 2 + 5 + 1 - 9 + 4

Note:

3 <= A.length <= 50000
-10000 <= A[i] <= 10000
"""
from typing import List


class Solution:
    def canThreePartsEqualSum(self, A: List[int]) -> bool:
        s = sum(A)
        if s % 3 != 0:
            return False

        target = s // 3
        count = 0
        cur_sum = 0
        for a in A:
            cur_sum += a
            if cur_sum == target:
                count += 1
                cur_sum = 0
            # elif cur_sum > target:
            #     return False
            # can have negative number 

        return count == 3 and cur_sum == 0


if __name__ == "__main__":
    assert Solution().canThreePartsEqualSum([3,3,6,5,-2,2,5,1,-9,4]) == True