Key Idea

• Pass start and path to DFS().
  • start denotes the index to start exploring from, initialized to 1.
  • It increments with each recursion to avoid reusing elements.
    • path keeps track of the current permutation being formed.
• The escape condition triggers when the length of path equals the desired combination length k.
• After each recursion, use pop() to backtrack and explore other combinations.

class Solution:
    def combine(self, n: int, k: int) -> List[List[int]]:
        def dfs(start, path):
            # Escape condition
            if len(path) == k:
                result.append(path.copy())
                return
            for value in range(start, n+1):
                path.append(value)
                dfs(value+1, path)  # Move to the next index in recursion
                path.pop()  # Backtrack

        result = []
        dfs(1, [])  # Start DFS from the beginning
        return result