We build combinations by picking elements from left to right. Starting from 1, at each step we either pick the current number or skip it, always moving forward.

1. Start with an empty combination and the number 1.
2. Add the current number and recurse for the next.
3. When the combination has k elements, save it.
4. Backtrack by removing the last number.

• Time Complexity: O(C(n, k) * k).
• Space Complexity: O(k).

def combine(n, k):
    result = []
    
    def backtrack(start, current):
        if len(current) == k:
            result.append(current[:])
            return
        for i in range(start, n + 1):
            current.append(i)
            backtrack(i + 1, current)
            current.pop()
    
    backtrack(1, [])
    return result