Two Numbers Appearing Once

Problem: All appear twice except two numbers. Find both.

$1.$ XOR all → $a \oplus b$

$2.$ Find set bit (differs between $a$, $b$)

$3.$ Partition by that bit, XOR each group

xorAll = XOR of all nums
diffBit = xorAll & (-xorAll)
a = b = 0
for num in nums:
    if num & diffBit: a ^= num
    else: b ^= num
return [a, b]

Numbers with same bit cancel in each partition.

Time: $O(n)$. Space: $O(1)$.