sorting - largest fromed number in a list python -

question: given list of non negative integers, arrange them such form largest number.

so given [1, 20, 23, 4, 8], largest formed number 8423201.

i couldn't understand following solution:

what num.sort(cmp=lambda x, y: cmp(y + x, x + y)) do?

and why have 2 parameters x , y? if input list, x , y represent in list?

class solution:     # @param num, list of integers     # @return string     def largestnumber(self, num):         num = [str(x) x in num]         num.sort(cmp=lambda x, y: cmp(y + x, x + y))         largest = ''.join(num)         return largest.lstrip('0') or '0'  if __name__ == "__main__":     num = [3, 30, 34, 5, 9]     print solution().largestnumber(num)  

can explain code solution? thanks.

python 2.x sort functions allowed cmp function comparing 2 items. cmp(a, b) returns -1 if < b, 0 if == b, or 1 if > b.

this code uses cmp "creatively" needed sort order solve problem; sort "8" before "80" because "880" > "808".

the problem want reverse alphabetical sort, want short strings before long ones (if prefixes identical).

a more general solution reverse-sort alphabetically, right-pad strings same length - @ least long longest string sorting - character sorts "greater 9".

how choose such character? well,

ord("9")                                         # -> 57 print("".join(chr(ch) ch in range(57, 75))   # "9:;<=>?@abcdefghij" 

so "a" looks choice, , it's easy remember because it's 10 in hexadecimal.

then

def largest_number(nums):     # convert strings     nums = [str(i) in nums]      # find length of longest string     longest = max(len(s) s in nums)      # create function pad strings length     def padded(s, pad_char="a", length=longest):         return s + pad_char * (length - len(s))      # sort strings greatest-to-least according padded values     nums.sort(key=padded, reverse=true)     # nums ["9", "5", "3", "34", "30"]      # resulting output-string     num = "".join(nums)      # remove leading 0s     #  (this should ever occur if input nums 0)     num = num.lstrip("0")      if num:         return num     else:         # string 0s - return single 0         return "0"  if __name__ == "__main__":     nums = [3, 30, 34, 5, 9]     print(largest_number(nums))   # -> "9533430"