31 lines
1.1 KiB
Markdown
31 lines
1.1 KiB
Markdown
---
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title: permutations
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tags: array,recursion,advanced
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---
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Generates all permutations of an array's elements (contains duplicates).
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⚠️ **WARNING**: This function's execution time increases exponentially with each array element. Anything more than 8 to 10 entries will cause your browser to hang as it tries to solve all the different combinations.
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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 `length` equal to `2` or `1`.
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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 => [item, ...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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```js
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permutations([1, 33, 5]); // [ [ 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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