Count Even Number of Ones

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Your question is Count Even Number of Ones. Start with the requirements on the right.

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Problem

SAP Concur Expense can represent a sequence of validation flags as integers, where 1 means a rule was triggered and 0 means it was not. Given an array of integers, return True if the array contains an even number of values equal to 1; otherwise, return False.

You must count only values exactly equal to 1. Other integer values do not affect the result. The algorithm should process the array in one pass and use constant extra space.

Formal Specification

Implement has_even_ones(flags), where flags is a list of integers. Return a boolean indicating whether the number of occurrences of 1 in flags is even. An empty array contains zero ones, and zero is even.

Constraints

0 <= len(flags) <= 10^5
flags[i] is an integer
Values other than 1 must be ignored
Process the input in one left-to-right pass
Use O(1) additional space

Function Signature

def has_even_ones(flags):

Your solutionPython 3

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