solution2.py

class Solution:
    def threeEqualParts(self, A: List[int]) -> List[int]:
        S = ''.join([str(a) for a in A])
        cnt_1 = S.count('1')
        if cnt_1 == 0:return [0, 2]
        # each part should have the same number of '1'
        if cnt_1 % 3!=0:return [-1, -1]
        
        counter_1, valid_part = 0, ''
        for i, s in enumerate(S[::-1]):
            if s=='1':counter_1+=1
            valid_part += s
            if counter_1==cnt_1//3:break
        valid_part = valid_part[::-1]
        N = len(valid_part)
        
        first_second = S[:-N]
        # we can remove leading zeros by collecting this information
        idx_1 = first_second.find('1')
        if first_second[idx_1:].startswith(valid_part):
            second = first_second[idx_1:][N:]
            idx_1_ = second.find('1')
            if second[idx_1_:].startswith(valid_part):
                return [idx_1+N-1, idx_1+N+idx_1_+N]
        return [-1, -1]