# LC 2824. Count Pairs Whose Sum is Less than Target

### [Problem link](https://leetcode.com/problems/count-pairs-whose-sum-is-less-than-target/)

###### tags: `leedcode` `python` `easy`

Given a **0-indexed** integer array <code>nums</code> of length <code>n</code> and an integer <code>target</code>, return the number of pairs <code>(i, j)</code> 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:**

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

## Solution 1

class Solution:
    def countPairs(self, nums: List[int], target: int) -> int:
        n = len(nums)
        res = 0
        
        for i in range(1, n):
            for j in range(i):
                if nums[i] + nums[j] < target:
                    res += 1
                    
        return res   

>### Complexity
>n = nums.length
>| | Time Complexity | Space Complexity |
>| ----------- | --------------- | ---------------- |
>| Solution 1 | O($n^2$) | O(1) |

## Note
x