字符串算法之KMP算法

目标算法：KMP

应用范围：字符串匹配

算法代码：

#include<cstdio>
#include<cstring>
#include<string>
#include<iostream>
using namespace std;

const int maxn = 505;
const int Inf = 0x3f3f3f;
int Next[maxn];
char a[maxn];
char b[maxn];
int len_a,len_b;
int pos;
int t;

void get_next(char *b,int *next) //获得next数组
{
    int i = 0;
    int j = -1;
    next[0] = -1;  // 将next[0]初始化为-1
    while(i < len_b)
    {
        if(j == -1 || b[i] == b[j]) //如果j == -1或者前缀和后缀相等的话，前缀和后缀都加1
        {
            ++i;
            ++j;
            next[i] = j; //给next数组赋值

        }
        else
            j = next[j];//否则回溯到合适位置
    }
}

int kmp(char *a,char *b)
{
    int i = 0;
    int j = 0;
    while(i < len_a)
    {
        if(j == -1 || a[i] == b[j]) //如果两个字符串字符相同，则对应位置+1
        {
            i++;
            j++;
        }
        else
            j = Next[j]; // 否则回溯到合适位置
        if(j == len_b)
            return i - len_b + 1;
    }
    return -1;
}

int main()
{
    scanf("%d",&t);
    while(t--)
    {
        scanf("%s%s",&a,&b);
        len_a = strlen(a);
        len_b = strlen(b);
        get_next(b,Next);
        int ans = kmp(a,b);
        printf("%d\n",ans);
    }
}