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snippets/permutations.md

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+### permutations
+
+⚠️ **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.
+
+Generates all permutations of an array's elements (contains duplicates).
+
+Use recursion.
+For each element in the given array, create all the partial permutations for the rest of its elements.
+Use `Array.map()` to combine the element with each partial permutation, then `Array.reduce()` to combine all permutations in one array.
+Base cases are for array `length` equal to `2` or `1`.
+
+```js
+const permutations = arr => {
+  if (arr.length <= 2) return arr.length === 2 ? [arr, [arr[1], arr[0]]] : arr;
+  return arr.reduce(
+    (acc, item, i) =>
+      acc.concat(
+        permutations([...arr.slice(0, i), ...arr.slice(i + 1)]).map(val => [
+          item,
+          ...val,
+        ])
+      ),
+    []
+  );
+};
+```
+
+```js
+permutations([1, 33, 5]) // [ [ 1, 33, 5 ], [ 1, 5, 33 ], [ 33, 1, 5 ], [ 33, 5, 1 ], [ 5, 1, 33 ], [ 5, 33, 1 ] ]
+```