Hard data structure and algorithm practice puzzle.
Given two numbers, add them without using +. XOR gives you the sum with every carry ignored. AND, shifted left by 1, gives you the carries. Set the first number to the XOR and the second to the carries, and repeat until there are no carries left. That's how an adder circuit works in hardware.
Solve it three ways: snap the jigsaw pieces together, fill in the blanks, or type it from scratch against the tests. This one is free.
Every line of the solution comes with a note explaining why it is there, and a step by step replay shows the code running. At level 3, where you type it yourself, you can ask for an AI mock interview on this problem that ends with a hiring manager's scorecard.
Then grow it into a real project with the Software Engineer button: this puzzle becomes the Match Flag Store, a pipeline you can run and download. See every Software Engineer project.
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