python实现最长公共子序列

def lcs(a,b): 
  lena=len(a) 
  lenb=len(b) 
  c=[[0 for i in range(lenb+1)] for j in range(lena+1)] 
  flag=[[0 for i in range(lenb+1)] for j in range(lena+1)] 
  for i in range(lena): 
    for j in range(lenb): 
      if a[i]==b[j]: 
        c[i+1][j+1]=c[i][j]+1 
        flag[i+1][j+1]='ok' 
      elif c[i+1][j]>c[i][j+1]: 
        c[i+1][j+1]=c[i+1][j] 
        flag[i+1][j+1]='left' 
      else: 
        c[i+1][j+1]=c[i][j+1] 
        flag[i+1][j+1]='up' 
  return c,flag 
 
def printLcs(flag,a,i,j): 
  if i==0 or j==0: 
    return 
  if flag[i][j]=='ok': 
    printLcs(flag,a,i-1,j-1) 
    print(a[i-1],end='') 
  elif flag[i][j]=='left': 
    printLcs(flag,a,i,j-1) 
  else: 
    printLcs(flag,a,i-1,j) 
     
a='ABCBDAB' 
b='BDCABA' 
c,flag=lcs(a,b) 
for i in c: 
  print(i) 
print('') 
for j in flag: 
  print(j) 
print('') 
printLcs(flag,a,len(a),len(b)) 
print('')