1. XOR identity
Zero is the identity: x XOR 0 = x.
let result = 0Typorithm · easy
Single Number
▷ Interactive algorithm labsingle-numberbit-manipulationxorself-inversepairs-cancel
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Full lesson · 8 learning steps Open any exercise for focused practice
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Step 1 of 8 · intro card
Learn Single Number
Bit Manipulation
XOR every element; equal pairs cancel to zero, so the one unpaired value remains.
XOR is its own inverse: x XOR x = 0, and x XOR 0 = x.
Folding the whole array with XOR cancels every value that appears twice.
Only the element appearing once is left, found in one pass with O(1) memory.
Preparing algorithm lab…
01
x XOR x = 0, x XOR 0 = x.
02
Pairs cancel out.
03
One pass, O(1) space.
function singleNumber(nums) {
let result = 0;
for (const num of nums) {
result ^= num;
}
return result;
}Read the implementation
Connect each part of the code to the concept before you type.
2. Fold with XOR
XOR every value; pairs cancel to zero, leaving the unique one.
result ^= num3. The loner
After all cancellations, only the single number survives.
return resultA complete learning loop
01
Understand
Read the concept map and follow the code breakdown.
02
Reconstruct
Type the implementation, repair mistakes and work through every recall exercise.
03
Verify
Use the final check without hints; revisit weak concepts in short practice sessions.