疯子的算法总结(七) 字符串算法之 manacher 算法 O（N）解决回文串

有点像DP的思想，写写就会做。

#include<bits/stdc++.h>
using namespace std;
const int maxn=1e7+5;
char a[maxn];
char s[2*maxn];
int len[maxn*2];
int manachar(char *p)
{
    int le=strlen(p);
    for(int i=0;i<le;i++)
    {
        s[i*2+2]=p[i];
        s[i*2+1]='+';
        //cout<<s[i*2+1]<<s[i*2+2];
    }
    s[0]='^',s[le*2+1]='+';//选取不可能出现的字符即可。
    //预处理字符串成为一个奇数串；
    int Max=0,pos=0,ans=0;
    le=2*le+1;
    for(int i=1;i<=le;i++)
    {
        if(Max>i)
        {
            len[i]=min(len[2*pos-i],Max-i);
        }
        else  len[i]=1;
        while(s[i-len[i]]==s[i+len[i]])len[i]++;
        ans =max(len[i],ans);
        if(i+len[i]>Max)
        {
            Max=len[i]+i;
            pos=i;
        }
    }
    return ans-1;
}
int main()
{
    int cnt=0;
    while(~scanf(" %s",a)){
        if(a[0]=='E')break;
        printf("Case %d: %d\n",++cnt,manachar(a));
        memset(a,0,sizeof(a));
    }
    return 0;
}
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