Pointers Undefined Behavior C++ [duplicate]

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• Undefined behavior and sequence points 4 answers

int array[]  = { 1 , 2 , 3 };
int *pointer = a;
for ( int i  = 0; i < 3; i++ ){
    *pointer++ *= *pointer;
}

This code gets the square of all the array elements but the output differs from computer to another.

Why is this undefined behavior?