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.

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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 nums[0] + nums[1] = 0 < target?

-(0, 2) since 0 < 2 and nums[0] + nums[2] = 1 < target?

- (0, 4) since 0 < 4 and nums[0] + nums[4] = 0 < target?

Note that (0, 3) is not counted since nums[0] + nums[3] 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 nums[0] + nums[1] = -4 < target?

- (0, 3) since 0 < 3 and nums[0] + nums[3] = -8 < target?

- (0, 4) since 0 < 4 and nums[0] + nums[4] = -13 < target

- (0, 5) since 0 < 5 and nums[0] + nums[5] = -7 < target?

- (0, 6) since 0 < 6 and nums[0] + nums[6] = -3 < target?

- (1, 4) since 1 < 4 and nums[1] + nums[4] = -5 < target?

- (3, 4) since 3 < 4 and nums[3] + nums[4] = -9 < target?

- (3, 5) since 3 < 5 and nums[3] + nums[5] = -3 < target

- (4, 5) since 4 < 5 and nums[4] + nums[5] = -8 < target?

- (4, 6) since 4 < 6 and nums[4] + nums[6] = -4 < target

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Constraints:

• 1 <= nums.length == n <= 50

• -50 <= nums[i], target <= 50

Runtime:?2 ms, faster than?100.00%?of?Java?online submissions for?Count Pairs Whose Sum is Less than Target.

Memory Usage:?41.5 MB, less than?50.00%?of?Java?online submissions for?Count Pairs Whose Sum is Less than Target.