字符串匹配

KMP

vector<int> GetNext(string &input){
    vector<int> nx(input.size() + 1,-1);
    int i = 0,j = -1,len = input.size();
    while(i < len){
        if(j == -1 || input[i] == input[j]){
            nx[++i] = ++j;
        }else j = nx[j];
    }
    return nx;
}

int GetIndex(string &a,string &b){
    vector<int> nx = GetNext(b);
    int i = 0,j = 0,alen = a.size(),blen = b.size();
    while(i < alen && j < blen){
        if(j == -1 || a[i] == b[j]) i++,j++;
        else j = nx[j];
    }
    if(j == blen) return i - j;
    return -1;
}

Rabin-Karp 算法

const int hashCode = 137;
void Gethash(string &input,vector<long long int> &h,vector<long long int> &p){
    h.assign(input.size() + 2,0);
    p.assign(input.size() + 2,1);

    for(int i = 0;i < input.size(); i++){
        h[i + 1] = int(h[i] * hashCode + input[i]);
        p[i + 1] = int(p[i] * hashCode);
    }
}

int GetResult(string &a,string &b){
    int n = a.size(),m = b.size();
    a.append(b);

    vector<long long int> h,p;
    Gethash(a,h,p);

    int r = a.size(),l = r - m + 1;
    int target = int(h[r] - h[l - 1] * p[r - l + 1]);
    
    for(int i = 1;i <= n;i++){
        int j = i + m - 1;
        int val = int(h[j] - h[i - 1] * p[j - i + 1]);
        if(val == target) return i - 1;
    }
    return -1;
}