Add two pointer approach

data_structures/arrays/sort_squares_of_an_array.rb

@@ -22,6 +22,7 @@ print(sorted_squares([4, -1, -9, 2]))
 def bubble_sort(array)
   array_length = array.size
   return array if array_length <= 1
+
   loop do
     swapped = false
     (array_length - 1).times do |i|

data_structures/arrays/two_sum.rb

@@ -1,4 +1,5 @@
 # Challenge name: Two Sum
+#
 # Given an array of integers nums and an integer target, return indices of the two numbers such that they add up to target.
 #
 # You may assume that each input would have exactly one solution, and you may not use the same element twice.
@@ -23,44 +24,52 @@
 # @param {Integer} target
 # @return {Integer[]}
 
-# 
+#
 # Approach 1: Brute Force with Addition
 #
+
+# Complexity analysis
+
 # Time Complexity: O(n^2). For each element, we try to find its complement
 # by looping through the rest of the array which takes O(n) time.
 # Therefore, the time complexity is O(n^2)
+
 # Space complexity: O(1)
-#
+
 def two_sum(nums, target)
   result_array = []
 
-  nums.each_with_index do |num, i|
-    nums.each_with_index do |num, j|
-      if i != j && i < j
-        current_sum = nums[i] + nums[j]
-        if current_sum == target
-          return [i, j]
-        end
-      end
+  nums.count.times do |i|
+    nums.count.times do |j|
+      next unless i != j && i < j
+
+      current_sum = nums[i] + nums[j]
+
+      return [i, j] if current_sum == target
     end
   end
 end
 
-print two_sum([2, 7, 11, 15], 9) 
+print(two_sum([2, 7, 11, 15], 9))
 # => [0,1]
-print two_sum([3, 2, 4], 6)
+
+print(two_sum([3, 2, 4], 6))
 # => [1,2]
-print two_sum([3, 3], 6)
+
+print(two_sum([3, 3], 6))
 # => [0,1]
 
 #
 # Approach 2: Brute Force with Difference
 #
+# Complexity analysis
+#
 # Time Complexity: O(N^2), where N is the length of the array
 #
 def two_sum(nums, target)
-  nums.each_with_index do |num, i|
+  nums.count.times do |i|
     target_difference = target - nums[i]
+
     nums.each_with_index do |num, j|
       if i != j && num == target_difference
         return [i, j]
@@ -69,38 +78,102 @@ def two_sum(nums, target)
   end
 end
 
-print two_sum([2, 7, 11, 15], 9) 
+print(two_sum([2, 7, 11, 15], 9))
 # => [0,1]
-print two_sum([3, 2, 4], 6)
+
+print(two_sum([3, 2, 4], 6))
 # => [1,2]
-print two_sum([3, 3], 6)
+
+print(two_sum([3, 3], 6))
 # => [0,1]
 
 #
 # Approach 3: Using a Hash
 #
+
+# Complexity analysis
+
 # Time complexity: O(n). We traverse the list containing n elements exactly twice.
 # Since the hash table reduces the lookup time to O(1), the time complexity is O(n).
 
 # Space complexity: O(n). The extra space required depends on the number of items
 # stored in the hash table, which stores exactly n elements.
-#
+
 def two_sum(nums, target)
   hash = {}
+
   # create a hash to store values and their indices
   nums.each_with_index do |num, i|
     hash[num] = i
   end
+
   # iterate over nums array to find the target (difference between sum target and num)
   nums.each_with_index do |num, i|
     difference_target = target - num
-    return [i, hash[difference_target]] if hash[difference_target] && hash[difference_target] != i
+
+    if hash[difference_target] && hash[difference_target] != i
+      return [i, hash[difference_target]]
+    end
   end
 end
 
-print two_sum([2, 7, 11, 15], 9) 
+nums = [2, 7, 11, 15]
+target = 9
+print(two_sum(nums, target))
 # => [0,1]
-print two_sum([3, 2, 4], 6)
+
+nums = [3, 2, 4]
+target = 6
+print(two_sum(nums, target))
 # => [1,2]
-print two_sum([3, 3], 6)
+
+nums = [3, 3]
+target = 6
+print(two_sum(nums, target))
+# => [0,1]
+
+#
+# Approach 4: Two pointers
+#
+
+# Complexity analysis
+
+# Time complexity: O(n). Each of the n elements is visited at
+# most once, thus the time complexity is O(n).
+
+# Space complexity: O(1). We only use two indexes, the space
+# complexity is O(1).
+
+def two_sum(numbers, target)
+  i = 0
+  j = numbers.length - 1
+  # note: sorting the array is important
+  numbers = numbers.sort
+
+  while i < j
+    sum = numbers[i] + numbers[j]
+
+    if target < sum
+      j -= 1
+    elsif target > sum
+      i += 1
+    else
+      return [i, j]
+    end
+  end
+end
+
+nums = [2, 7, 11, 15]
+target = 9
+print(two_sum(nums, target))
+# => [0,1]
+
+nums = [2, 3, 4]
+target = 6
+print(two_sum(nums, target))
+# => [0,2]
+
+nums = [-1, 0]
+target = -1
+print(two_sum(nums, target))
 # => [0,1]