2824 - Count Pairs Whose Sum is Less than Target.

Easy

Given a 0-indexed integer array nums of length n and an integer target, return the number of pairs (i, j) where 0 <= i < j < n and nums[i] + nums[j] < target.

Example 1:

Input: nums = -1,1,2,3,1, target = 2

Output: 3

Explanation:

There are 3 pairs of indices that satisfy the conditions in the statement:

• (0, 1) since 0 < 1 and nums0 + nums1 = 0 < target

• (0, 2) since 0 < 2 and nums0 + nums2 = 1 < target

• (0, 4) since 0 < 4 and nums0 + nums4 = 0 < target

Note that (0, 3) is not counted since nums0 + nums3 is not strictly less than the target.

Example 2:

Input: nums = -6,2,5,-2,-7,-1,3, target = -2

Output: 10

Explanation:

There are 10 pairs of indices that satisfy the conditions in the statement:

• (0, 1) since 0 < 1 and nums0 + nums1 = -4 < target

• (0, 3) since 0 < 3 and nums0 + nums3 = -8 < target

• (0, 4) since 0 < 4 and nums0 + nums4 = -13 < target

• (0, 5) since 0 < 5 and nums0 + nums5 = -7 < target

• (0, 6) since 0 < 6 and nums0 + nums6 = -3 < target

• (1, 4) since 1 < 4 and nums1 + nums4 = -5 < target

• (3, 4) since 3 < 4 and nums3 + nums4 = -9 < target

• (3, 5) since 3 < 5 and nums3 + nums5 = -3 < target

• (4, 5) since 4 < 5 and nums4 + nums5 = -8 < target

• (4, 6) since 4 < 6 and nums4 + nums6 = -4 < target

Constraints:

• 1 <= nums.length == n <= 50

• -50 <= nums[i], target <= 50