Finding a number that repeats even no of times where all the other numbers repeat odd no of times

Question

Given is an array of integers. Each number in the array repeats an ODD number of times, but only 1 number is repeated for an EVEN number of times. Find that number.

Answer 1

You don't need to keep the number of times each element is found - just whether it's even or odd number of time - so you should be ok with 1 bit for each element. Start with 0 for each element, then flip the corresponding bit when you encounter the element. Next time you encounter it, flip the bit again. At the end, just check which bit is 1.