问构造一个示例，其中KMP算法必须多次回溯。

EN

def computeLPSArray(pat, M, lps):
    len = 0 # length of the previous longest prefix suffix
    lps[0]=0 # lps[0] is always 0
    i = 1
    while i < M:
    if pat[i]==pat[len]:
        len+=1
        lps[i] = len
        i+=1
    else:
        if len!=0:
            # backtrack section - When will we get here 3-4 times for the same i??? 
            len = lps[len-1]     
        else:
            lps[i] = 0
            i+=1