36 lines
1.2 KiB
Markdown
36 lines
1.2 KiB
Markdown
---
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title: permutations
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tags: array,algorithm,recursion,advanced
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firstSeen: 2018-02-19T15:47:47+02:00
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lastUpdated: 2020-12-28T13:49:24+02:00
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---
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Generates all permutations of an array's elements (contains duplicates).
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- Use recursion.
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- For each element in the given array, create all the partial permutations for the rest of its elements.
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- Use `Array.prototype.map()` to combine the element with each partial permutation, then `Array.prototype.reduce()` to combine all permutations in one array.
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- Base cases are for `Array.prototype.length` equal to `2` or `1`.
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- ⚠️ **WARNING**: This function's execution time increases exponentially with each array element. Anything more than 8 to 10 entries may cause your browser to hang as it tries to solve all the different combinations.
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```js
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const permutations = arr => {
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if (arr.length <= 2) return arr.length === 2 ? [arr, [arr[1], arr[0]]] : arr;
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return arr.reduce(
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(acc, item, i) =>
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acc.concat(
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permutations([...arr.slice(0, i), ...arr.slice(i + 1)]).map(val => [
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item,
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...val,
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])
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),
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[]
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);
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};
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```
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```js
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permutations([1, 33, 5]);
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// [ [1, 33, 5], [1, 5, 33], [33, 1, 5], [33, 5, 1], [5, 1, 33], [5, 33, 1] ]
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```
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