【kmp算法】C++代码实现

推荐先在B站搜索KMP看一个印度小哥讲解的视频：

https://www.bilibili.com/video/BV1Ys411d7yh

【强烈推荐，看一两遍就懂了】

代码借鉴：

http://blog.csdn.net/weixin_41106545/article/details/83573340

//KMP substring search

#include <iostream>
#include<vector>
#include<string>
using namespace std;

//以下即NEXT数组的实现
vector<int> getTempArray(string pattern){
    vector<int> result;//声明返回的vector
    if(pattern.size()==0 || pattern.size()==1){
        return result;
        //如果字符串的长度为0，直接返回空的vector
        //如果字符串的长度为1，直接返回此时的vector
    } 
    
    result.push_back(0);//首先压入0
    
    int j=0;
    int i=1;
    
    while(j < pattern.size()-1 && i <= pattern.size()-1){
        if(pattern[i] == pattern[j]){
            result.push_back(j+1);
            i++;
            j++;
        }
        else{
            if(j==0){
                result.push_back(0);
                i++;
            }
            else{
                while(j>=1 && pattern[i] != pattern[j]){
                    j = result[j-1];
                }
                if(pattern[i] == pattern[j]){
                    result.push_back(j+1);
                    i++;
                    j++;
                }
                else{
                    result.push_back(0);
                    j++;
                }
            }
        }
    }
    return result;
}

int kmp (string t, string p)//t:text p:pattern
{
    vector<int> next = getTempArray(p);
    int j=0;
    for (int i=0; i<t.length(); i++){
        while(j>0 && t[i] != p[j]){
            j=next[j-1];
        }
        if(t[i] == p[j]){
            j++;
        }
        if(j==p.length()){
            return i-p.length()+1;
        }
    } 
    return -1;
}

int main(){
    string str;
    cout << "Enter the string:";
    cin >> str;
    
    string pattern;
    cout << "Enter the pattern:";
    cin >> pattern;
    
    int result = kmp(str,pattern);
    if (result == -1){
        cout << "Can't find pattern in the string!\n";
    }
    else{
        cout << result << endl;
    }
    
}