pandas.melt(frame, id_vars=None, value_vars=None, var_name=None, value_name='value', col_level=None)

import pandas as pd
df = pd.DataFrame({'A': {0: 'a', 1: 'b', 2: 'c'},
                   'B': {0: 1, 1: 3, 2: 5},
                   'C': {0: 2, 1: 4, 2: 6}
                   })
df
'''
   A  B  C
0  a  1  2
1  b  3  4
2  c  5  6
'''

pd.melt(df, id_vars=['A'], value_vars=['B'])
'''
   A variable  value
0  a        B      1
1  b        B      3
2  c        B      5
'''

pd.melt(df, id_vars=['A'], value_vars=['B', 'C'])
'''
   A variable  value
0  a        B      1
1  b        B      3
2  c        B      5
3  a        C      2
4  b        C      4
5  c        C      6
'''

pd.melt(df, id_vars=['A'], value_vars=['B'],
        var_name='myVarName', value_name='myValueName')
'''
   A myVarName  myValueName
0  a         B            1
1  b         B            3
2  c         B            5
'''

pd.melt(df, id_vars=['A'], value_vars=['B', 'C'],
        ignore_index=False)
'''
   A variable  value
0  a        B      1
1  b        B      3
2  c        B      5
0  a        C      2
1  b        C      4
2  c        C      6
'''

# 多重索引
df.columns = [list('ABC'), list('DEF')]
df
'''
   A  B  C
   D  E  F
0  a  1  2
1  b  3  4
2  c  5  6
'''

# 选择最外层索引
pd.melt(df, col_level=0, id_vars=['A'], value_vars=['B'])
'''
   A variable  value
0  a        B      1
1  b        B      3
2  c        B      5
'''

# 选择内层索引
pd.melt(df, col_level=1, id_vars=['D'], value_vars=['E'])

# 选择复合索引
pd.melt(df, id_vars=[('A', 'D')], value_vars=[('B', 'E')])
'''
  (A, D) variable_0 variable_1  value
0      a          B          E      1
1      b          B          E      3
2      c          B          E      5
'''

class Solution:
    def countPrimes(self, n: int) -> int:
        counts = 0
        for i in range(n):
            if self.isprime(i):
                counts += 1
        return counts
                
    def isprime(self,n):
        from itertools import count
        if n <=1:
            return False
        for i in count(2):
            if i* i > n:
                return True
            if n % i == 0:
                return False

class Solution:
    def largestNumber(self, nums: List[int]) -> str:
        class Comapre(str):
            def __lt__(self,other):
                return int(self+other) > int(other+self)
        nums.sort(key=Comapre)
        return str(int(''.join(map(str,nums))))