77. Combinations

Author: remy727

README.md

@@ -59,6 +59,7 @@
 | 73 | Set Matrix Zeroes | [Ruby](./algorithms/ruby/0073-set-matrix-zeroes.rb) | Medium |
 | 74 | Search a 2D Matrix | [Ruby](./algorithms/ruby/0074-search-a-2d-matrix.rb) | Medium |
 | 76 | Minimum Window Substring | [Ruby](./algorithms/ruby/0076-minimum-window-substring.rb) | Hard |
+| 77 | Combinations | [Ruby](./algorithms/ruby/0077-combinations.rb) | Medium |
 | 80 | Remove Duplicates from Sorted Array II | [Ruby](./algorithms/ruby/0080-remove-duplicates-from-sorted-array-ii.rb) | Medium |
 | 82 | Remove Duplicates from Sorted List II | [Ruby](./algorithms/ruby/0082-remove-duplicates-from-sorted-list-ii.rb) | Medium |
 | 86 | Partition List | [Ruby](./algorithms/ruby/0086-partition-list.rb) | Medium |

algorithms/ruby/0077-combinations.rb

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+# frozen_string_literal: true
+
+# 77. Combinations
+# https://leetcode.com/problems/combinations
+# Medium
+
+=begin
+Given two integers n and k, return all possible combinations of k numbers chosen from the range [1, n].
+
+You may return the answer in any order.
+
+Example 1:
+Input: n = 4, k = 2
+Output: [[1,2],[1,3],[1,4],[2,3],[2,4],[3,4]]
+Explanation: There are 4 choose 2 = 6 total combinations.
+Note that combinations are unordered, i.e., [1,2] and [2,1] are considered to be the same combination.
+
+Example 2:
+Input: n = 1, k = 1
+Output: [[1]]
+Explanation: There is 1 choose 1 = 1 total combination.
+
+Constraints:
+* 1 <= n <= 20
+* 1 <= k <= n
+=end
+
+# @param {Integer} n
+# @param {Integer} k
+# @return {Integer[][]}
+def combine(n, k)
+  return (1..n).map { |i| [i] } if k == 1
+
+  i = n
+  result = []
+
+  while i > 1
+    result += combine(i - 1, k - 1).map { |tuple| tuple + [i] }
+    i -= 1
+  end
+
+  result
+end
+
+# **************** #
+#       TEST       #
+# **************** #
+
+require "test/unit"
+class Test_combine < Test::Unit::TestCase
+  def test_
+    assert_equal [[1, 2], [1, 3], [1, 4], [2, 3], [2, 4], [3, 4]].sort, combine(4, 2).sort
+    assert_equal [[1]], combine(1, 1)
+  end
+end