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##### ###### ##### ### # # ### # # ###### ## ## ## ## ## ## ## # # # # # ## ##### #### ##### # # # # # # # #### ## # ## ## ## ## # # # # # ## ## # ###### ## ### # ### # ######
XOR all elements. Result is where and are the two unique numbers.
Find any set bit in this XOR (the rightmost is easiest: xor & (-xor)).
This bit differs between and . Partition all numbers by this bit.
XOR within each group. One group gives , the other gives .
