1. 全排列（無重復元素）

核心思想：交換法避免額外空間

def permute(nums):def backtrack(first=0):if first == len(nums):res.append(nums.copy())returnfor i in range(first, len(nums)):nums[first], nums[i] = nums[i], nums[first]backtrack(first + 1)nums[first], nums[i] = nums[i], nums[first]res = []backtrack()return res

2. 全排列II（含重復元素）

關鍵點：剪枝去重

def permuteUnique(nums):nums.sort()res = []def backtrack(path, used):if len(path) == len(nums):res.append(path.copy())returnfor i in range(len(nums