计算字符串的相似度

#CSD means CalculateStringDistance
def CSD(StrA,ABegin,AEnd,StrB,BBegin,BEnd):
    global Memory
    if ABegin > AEnd:
        if BBegin > BEnd:
            return 0 # StrA and StrB are all empty
        else:
            return BEnd-BBegin+1 #return StrB's len
    if BBegin > BEnd:
        if ABegin > AEnd:
            return 0
        else:
            return AEnd-ABegin+1

    if StrA[ABegin]==StrB[BBegin]:
        return CSD(StrA,ABegin+1,AEnd,StrB,BBegin+1,BEnd)
    else:
        #avoid excess computing
        if Memory[ABegin][BBegin+1] != -1:
            t1=Memory[ABegin][BBegin+1]
        else:
            t1=CSD(StrA,ABegin,AEnd,StrB,BBegin+1,BEnd)
            Memory[ABegin][BBegin+1]=t1
            
        if Memory[ABegin+1][BBegin] != -1:
            t2=Memory[ABegin][BBegin+1]
        else:
            t2=CSD(StrA,ABegin+1,AEnd,StrB,BBegin,BEnd)
            Memory[ABegin][BBegin+1]=t2
            
        if Memory[ABegin+1][BBegin+1] != -1:
            t3=Memory[ABegin+1][BBegin+1]
        else:
            t3=CSD(StrA,ABegin+1,AEnd,StrB,BBegin+1,BEnd)
            Memory[ABegin][BBegin+1]=t3

        
        """t1=CBD(StrA,ABegin,AEnd,StrB,BBegin+1,BEnd) #delete
        t2=CBD(StrA,ABegin+1,AEnd,StrB,BBegin,BEnd) #delete
        t3=CBD(StrA,ABegin+1,AEnd,StrB,BBegin+1,BEnd) #modify"""
        return min(t1,t2,t3)+1

#test
StrA="tieba"
StrB="iba"
ABegin=0
AEnd=len(StrA)-1
BBegin=0
BEnd=len(StrB)-1

Memory=[[(-1) for col in range(BBegin,BEnd+3)] for row in range(ABegin,AEnd+3)] #set bigger to avoid IndexError

print(CSD(StrA,ABegin,AEnd,StrB,BBegin,BEnd))