不懂怎样才能写出时间复杂度为O(n)，我只能写O(n^2) 
#include <stdio.h>
#include <string.h>

int main(){
    char s[350]={0};
    scanf("%s",s);
    int slen=strlen(s),max=1,j,k,len=0;
    for(int i=0;i<slen;i++){
        if(s[i]!=s[i+1] || s[i-1]==s[i+1]){//奇对称
            j=i-1,k=i+1;
            while(j>-1 && k<slen){//以该数为中心向两端查找
                if(s[j]==s[k]){
                    len=k-j+1;
                    j--;
                    k++;
                    if(len>max){
                        max=len;
                    }
                }
                else{
                    len=0;
                    break;
                }
            }
        }
        if(s[i]==s[i+1]){//偶对称
            j=i-1,k=i+2;
            if(max<2){
                max=2;
            }
            while(j>-1 && k<slen){//以该数为中心向两端查找
                if(s[j]==s[k]){
                    len=k-j+1;
                    j--;
                    k++;
                    if(len>max){
                        max=len;
                    }
                }
                else{
                    len=0;
                    break;
                }
            }
        }
    }
    printf("%d",max);
}