Fixes #9943 (#10252)

* added doctest for all_permutations.py

* added doctest for all_subsequences.py

* added doctest for all_subsequences.py

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* doctest added

* updated

* Update backtracking/all_subsequences.py

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Co-authored-by: Harsh Buddhdev <[email protected]>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Tianyi Zheng <[email protected]>