Leetcode problem 49 - Count Pairs Whose Sum is Less than Target

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 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

Constraints:

1 <= nums.length == n <= 50 -50 <= nums[i], target <= 50

/**
 * @param {number[]} nums
 * @param {number} target
 * @return {number}
 */
var countPairs = function (nums, target) {
  let count = 0;
  for (let i = 0; i < nums.length; i++) {
    for (let j = i + 1; j < nums.length; j++) {
      if (nums[i] + nums[j] < target) {
        count++;
      }
    }
  }
  return count;
};

It counts and returns the number of pairs of elements from the nums array whose sum is less than the given target. The function achieves this by using two nested loops to compare each pair of elements in the array. For each pair, it checks if their sum is less than the target, and if so, it increments the count variable. After processing all possible pairs, the function returns the final count, indicating how many pairs meet the specified condition.