Add string permutation algorithm.

src/algorithms/string/permutations/README.md

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+# String Permutations
+
+A permutation, also called an “arrangement number” or “order”, is a rearrangement of 
+the elements of an ordered list `S` into a one-to-one correspondence with `S` itself. 
+A string of length `n` has `n!` permutation.
+
+Below are the permutations of string `ABC`.
+
+`ABC ACB BAC BCA CBA CAB`

src/algorithms/string/permutations/__test__/permutateString.test.js

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+import permutateString from '../permutateString';
+
+describe('permutateString', () => {
+  it('should permutate string', () => {
+    const permutations0 = permutateString('');
+    expect(permutations0).toEqual([]);
+
+    const permutations1 = permutateString('A');
+    expect(permutations1).toEqual(['A']);
+
+    const permutations2 = permutateString('AB');
+    expect(permutations2.length).toBe(2);
+    expect(permutations2).toEqual([
+      'BA',
+      'AB',
+    ]);
+
+    const permutations6 = permutateString('AA');
+    expect(permutations6.length).toBe(2);
+    expect(permutations6).toEqual([
+      'AA',
+      'AA',
+    ]);
+
+    const permutations3 = permutateString('ABC');
+    expect(permutations3.length).toBe(2 * 3);
+    expect(permutations3).toEqual([
+      'CBA',
+      'BCA',
+      'BAC',
+      'CAB',
+      'ACB',
+      'ABC',
+    ]);
+
+    const permutations4 = permutateString('ABCD');
+    expect(permutations4.length).toBe(2 * 3 * 4);
+    expect(permutations4).toEqual([
+      'DCBA',
+      'CDBA',
+      'CBDA',
+      'CBAD',
+      'DBCA',
+      'BDCA',
+      'BCDA',
+      'BCAD',
+      'DBAC',
+      'BDAC',
+      'BADC',
+      'BACD',
+      'DCAB',
+      'CDAB',
+      'CADB',
+      'CABD',
+      'DACB',
+      'ADCB',
+      'ACDB',
+      'ACBD',
+      'DABC',
+      'ADBC',
+      'ABDC',
+      'ABCD',
+    ]);
+
+    const permutations5 = permutateString('ABCDEF');
+    expect(permutations5.length).toBe(2 * 3 * 4 * 5 * 6);
+  });
+});

src/algorithms/string/permutations/permutateString.js

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+export default function permutateString(str) {
+  if (str.length === 0) {
+    return [];
+  }
+
+  if (str.length === 1) {
+    return [str];
+  }
+
+  const permutations = [];
+
+  // Get all permutations of string of length (n - 1).
+  const previousString = str.substring(0, str.length - 1);
+  const previousPermutations = permutateString(previousString);
+
+  // Insert last character into every possible position of every previous permutation.
+  const lastCharacter = str.substring(str.length - 1);
+
+  for (
+    let permutationIndex = 0;
+    permutationIndex < previousPermutations.length;
+    permutationIndex += 1
+  ) {
+    const currentPermutation = previousPermutations[permutationIndex];
+
+    // Insert strLastCharacter into every possible position of currentPermutation.
+    for (let positionIndex = 0; positionIndex <= currentPermutation.length; positionIndex += 1) {
+      const permutationPrefix = currentPermutation.substr(0, positionIndex);
+      const permutationSuffix = currentPermutation.substr(positionIndex);
+      permutations.push(permutationPrefix + lastCharacter + permutationSuffix);
+    }
+  }
+
+  return permutations;
+}