[Python] Is there a NumPy magic to generate this array?

I have an array a of increasing indexes, e.g. [2 5 9 10], which indicates positions of value change. Assuming the output values are 0 and 1, I want to get array b:

[0 0 1 1 1 0 0 0 0 1 0]


Is there a NumPy magic to transform a into b?

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