2824

Author: theforestvn88

2801-2900/2824_count_pairs_whose_sum_is_less_than_target.rb

@@ -0,0 +1,32 @@
+# @param {Integer[]} nums
+# @param {Integer} target
+# @return {Integer}
+def count_pairs(nums, target)
+    count = 0
+    (0...nums.size).each { |i|
+        (i+1...nums.size).each { |j|
+            count += 1 if nums[i] + nums[j] < target 
+        }
+    }
+
+    count
+end
+
+# sort + binary-search
+def count_pairs(nums, target)
+    # the condition i < j does not matter (but the condition i < j, a < b will matter)
+    # assume that we have pair a, b that a + b < target, at _i, _j respectively,
+    # in case of a before b, we have i = _i < j = _j
+    # in case if a after b, we have i = _j < j = _i
+    # so we always can count pair (a,b) (a + b < target) no matter what the order of a, b is
+    # so we can sort the nums and take advantage of it
+    nums.sort!
+    nums << Float::INFINITY
+    count = 0
+    (0...nums.size).each { |i|
+        j = nums.bsearch_index { |x| x >= target - nums[i] }
+        count += [i-1, j-1].min + 1 if j
+    }
+
+    count
+end