LeetCode Combinations and Permutations Algorithm in JavaScript

There are a lot of Eureka moments when doing the algorithm problems. The first time I heard combinations and permutations can be solved by tree traversal is definitely one of the most memorable moment.

How does that work?

The image above is how we get all the combinations. I think the first root vertice “12345” should not be in the tree, but anyway, it’s enough for us to get the idea. As you can see, from left to right, the sub-trees are getting smaller because under the vertice “2”, we don’t have “1” anymore, and under “3” we don’t have “2” and “1”. That’s something we need to pay attention since it’s different from permutations.

For permutation, we do want to have “1” again under “2”, but if any number has been used under “1”, we don’t want to use it again under “2”. So, we need to track which are used.

Let’s use some LeetCode problems to practice the code…

The tricky part is start, which helps us skip the candidates we’ve already used in other combinations.

Instead of start, permutations use used to track any nums already used in the same prefix.

More Combinations Problems:

More Permutations Problems:

Ref:

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