回溯算法与列表推导式有什么区别？

ans = []
def trace(path,choices):
    if len(path) == len(nums):
        ans.append(list(path))

    for i in choices:
        if i in path:       
            continue
        path.append(i)
        trace(path,choices)
        path.pop()      
    return ans
nums = [1,2,3]
print(trace([],nums))
>>> [[1, 2, 3], [1, 3, 2], [2, 1, 3], [2, 3, 1], [3, 1, 2], [3, 2, 1]]

res = []  
path = []  
def bt(nums,startIndex):
    res.append(path[:])
    for i in range(startIndex,len(nums)):
        path.append(nums[i])
        bt(nums,i+1)
        path.pop()
nums = [1,2,3]
bt(nums,0)
print(res)
>>> [[], [1], [1, 2], [1, 2, 3], [1, 3], [2], [2, 3], [3]]