Adding the 4-sum problem

[v1shant]

Detailed description

I want to add the 4Sum problem from leetcode.
https://leetcode.com/problems/4sum/

Context

It an important 2-pointer question and is present in lots of placement sheets

Possible implementation

No response

Additional information

No response

[shlokam]

@v1shant Can you assign this to me?

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