function遇到return就會結束這個function:

class Solution(object):
    def permute(self, nums):
        result = []
        path = []
        def backtrace(nums, index):
            if len(path) > len(nums):
                return  #跳出backtrace
            if len(path) == len(nums):
                result.append(path[:])
                return  #跳出backtrace
            for i in range(index, len(nums)):
                if nums[i] in path:
                    continue #跳過一次迴圈執行
                path.append(nums[i])
                backtrace(nums, 0)
                path.pop()
        backtrace(nums, 0)
        return result

if len(path) > len(nums):
if len(path) == len(nums):
這兩個條件滿足任一都會，並且第一個優先
不滿足這兩個條件的都會執行下面的for

然後if nums[i] in path:
會跳過一次執行。

附帶一提，我個人比較愛這樣寫

for i in range(index, len(nums)):
    if nums[i] not in path:
        path.append(nums[i])
        backtrace(nums, 0)
        path.pop()

for迴圈裏面有個if nums[i] in path,
如果滿足條件就會跳過這次迴圈.