查找使用列表表示的所有组合

def find_combinations(arr, n, r):
    if r == 0:
        return [[]]
    if len(arr) == r:
        return [arr]
    
    # Include the first element and recurse
    include_first = find_combinations(arr[1:], n - 1, r - 1)
    # Exclude the first element and recurse
    exclude_first = find_combinations(arr[1:], n - 1, r)
    
    return include_first + exclude_first

# 示例使用
arr = [1, 2, 3]
r = 2
combinations = find_combinations(arr, len(arr), r)
print(combinations)

def find_combinations_optimized(arr, n, r, start=0, current=[]):
    if len(current) == r:
        return [current[:]]
    
    combinations = []
    for i in range(start, n):
        current.append(arr[i])
        combinations.extend(find_combinations_optimized(arr, n, r, i + 1, current))
        current.pop()
    
    return combinations

# 示例使用
arr = [1, 2, 3]
r = 2
combinations = find_combinations_optimized(arr, len(arr), r)
print(combinations)