Given a 0-indexed integer array nums of size n and two integers lower and upper, return the number of fair pairs.

To solve the problem of counting "fair pairs" within an integer array, we'll outline the steps necessary to arrive at the solution.

1. Break Down the Problem

1. Define what a "fair pair" is: A pair (i, j) is considered fair if i < j and lower <= nums[i] + nums[j] <= upper.
2. Iterate through all possible pairs in the array while checking the fairness condition.

2. Relevant Concepts

The core concepts to utilize here include:

• Nested loops to generate pairs of indices.
• Conditional checks to ensure the pairs meet the specified conditions.

3. Analysis and Detail

1. Create a counter variable initialized to 0 to keep track of fair pairs.
2. Use two nested loops:
   • The outer loop runs from 0 to n - 2 (to get the first element of the pair).
   • The inner loop runs from the outer loop index plus 1 to n - 1 (to get the second element of the pair).
3. For each pair of indices (i, j), calculate the sum nums[i] + nums[j].
4. Check if this sum falls within the range defined by lower and upper. If it does, increment the counter.

4. Verify and Summarize

After iterating through all possible pairs, the counter will represent the total number of fair pairs. Ensure that you've covered all indices properly.

Final Answer

The function implementation in Python can be as follows:

def countFairPairs(nums, lower, upper):
    count = 0
    n = len(nums)
    
    for i in range(n - 1):
        for j in range(i + 1, n):
            sum_pair = nums[i] + nums[j]
            if lower <= sum_pair <= upper:
                count += 1
                
    return count

In this implementation, you can call the function countFairPairs(nums, lower, upper) with your specific input values to get the number of fair pairs.