Find permutation of string.

Author: deepakhb2

Algorithm-Recursions/permutation.rb

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+=begin
+Write a program to print all permutations of a given string
+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.
+Source: Mathword(http://mathworld.wolfram.com/Permutation.html)
+
+Below are the permutations of string ABC.
+ABC ACB BAC BCA CBA CAB
+=end
+
+# Time complexity: O(2^n)
+def permutation(arr, l, r)
+  if(l == r)
+    p arr.join.to_s
+  else
+    for i in (l..r)
+      arr[i], arr[l] = arr[l], arr[i]
+      permutation(arr, l+1, r)
+      arr[i], arr[l] = arr[l], arr[i]
+    end
+  end
+end
+
+a = ['a', 'b', 'c']
+permutation(a, 0, a.size-1)