Solution 015, 019 and 020 in Ruby

solution/001.Two Sum/Solution.rb

@@ -8,4 +8,4 @@ def two_sum(nums, target)
     end
     next
   end
-end
+end

solution/002.Add Two Numbers/Solution.rb

@@ -34,4 +34,4 @@ def add_two_numbers(l1, l2)
   end
 
   l3
-end
+end

solution/007.Reverse Integer/Solution.rb

@@ -18,4 +18,4 @@ def reverse(x)
   # have to explicitly constraint the int boundary as per the dummy test case
   res = s.to_i
   res <= 214_748_364_7 && res >= -214_748_364_8 ? res : 0
-end
+end

solution/013.Roman to Integer/Solution.rb

@@ -29,4 +29,4 @@ def roman_to_int(s)
   end
 
   res
-end
+end

solution/014.Longest Common Prefix/Solution.rb

@@ -22,4 +22,4 @@ def longest_common_prefix(strs)
   end
 
   idx > 0 ? strs[0][0..idx] : ''
-end
+end

solution/015.3Sum/Solution.rb

@@ -0,0 +1,28 @@
+# @param {Integer[]} nums
+# @return {Integer[][]}
+def three_sum(nums)
+  res = []
+  nums.sort!
+
+  for i in 0..(nums.length - 3)
+    next if i > 0 && nums[i - 1] == nums[i]
+    j = i + 1
+    k = nums.length - 1
+    while j < k do
+      sum = nums[i] + nums[j] + nums[k]
+      if sum < 0
+        j += 1
+      elsif sum > 0
+        k -= 1
+      else
+        res += [[nums[i], nums[j], nums[k]]]
+        j += 1
+        k -= 1
+        j += 1 while nums[j] == nums[j - 1]
+        k -= 1 while nums[k] == nums[k + 1]
+      end
+    end
+  end
+
+  res
+end

solution/019.Remove Nth Node From End of List/Solution.rb

@@ -0,0 +1,27 @@
+# Definition for singly-linked list.
+# class ListNode
+#     attr_accessor :val, :next
+#     def initialize(val)
+#         @val = val
+#         @next = nil
+#     end
+# end
+
+# @param {ListNode} head
+# @param {Integer} n
+# @return {ListNode}
+def remove_nth_from_end(head, n)
+  return nil if head.next.nil?
+  count = 1
+  q = head
+  s = head
+
+  while q.next do
+    q = q.next
+    s = s.next if count > n
+    count += 1
+  end
+  return head.next if count == n
+  s.next = s.next.next
+  head
+end

solution/020.Valid Parentheses/Solution.rb

@@ -0,0 +1,22 @@
+# @param {String} s
+# @return {Boolean}
+def is_valid(s)
+  stack = ''
+  s.split('').each do |c|
+    if ['{', '[', '('].include?(c)
+      stack += c
+    else
+      if c == '}' && stack[stack.length - 1] == '{'
+
+        stack = stack.length > 1 ? stack[0..stack.length - 2] : ""
+      elsif c == ']' && stack[stack.length - 1] == '['
+        stack = stack.length > 1 ? stack[0..stack.length - 2] : ""
+      elsif c == ')' && stack[stack.length - 1] == '('
+        stack = stack.length > 1 ? stack[0..stack.length - 2] : ""
+      else
+        return false
+      end
+    end
+  end
+  stack == ''
+end