字符串匹配

这里介绍两种常见算法, Brute Force和KMP. 

代码如下：

//字符串匹配
//Brute Force
class StringPattern {
private:
    string value;
public:
    StringPattern(const string &input) {
        value = input;
    }
    int match(const string &buffer){
        //注意当buffer.length()<value.length()时, buffer.length()-value.length()的值是一个大整数
        //所以需要用if判断
        if(buffer.length()<value.length()){
            return -1;
        }
        for(int i=0;i<=(buffer.length()-value.length());i++){
            int j=0;
            for(;j<value.length();j++){
                if(buffer[i+j]!=value[j]){
                    break;
                }
            }
            if(j==value.length()){
                return i;
            }
        }
        return -1;
    }
};

int main() {
    string str_buffer, str_pattern;
    cin >> str_pattern >> str_buffer;
    StringPattern pattern(str_pattern);
    int location = pattern.match(str_buffer);
    if (location != -1) {
        cout << "match success, location is " << location << endl;
    } else {
        cout << "match failed!" << endl;
    }
    return 0;
}









//KMP
#include <iostream>
#include <string>
using std::cin;
using std::cout;
using std::endl;
using std::string;

class StringPattern {
private:
    string value;
    //next记录每个char的上一个匹配位置
    int *next;
public:
    StringPattern(const string &input) {
        value = input;
        next = new int[input.length() + 1];
        get_next();
    }
    ~StringPattern() {
        delete[] next;
    }
    void get_next() {
        next[0]=-1;
        //第一个next记为-1,循环求之后的next
        for(int i=1,match=-1;i<value.length();i++){
            //如果当前match>-1但不匹配,则回到上一个匹配点,直到成功匹配或者回到-1
            while(match>=0&&value[match+1]!=value[i]){
                match=next[match];
            }
            //成功匹配
            if(value[match+1]==value[i]){
                    match++;
                }
            next[i]=match;
        }
    }
    //在buffer中找value
    int match(const string &buffer) const {
        //注意buffer的每一位都要检查
        for(int i=0,match=-1;i<buffer.length();i++){
            //如果当前match>-1但不匹配,则回到上一个匹配点,直到成功匹配或者回到-1
            while(match>=0&&value[match+1]!=buffer[i]){
                match=next[match];
            }
            if(value[match+1]==buffer[i]){
                match++;
            }
            if(match==value.length()-1){
                //这里也可以先不返回,改成记录下所有的匹配位置
                return i-match;
            }
        }
        //匹配失败
        return -1;
    }
};

int main() {
    string str_buffer, str_pattern;
    cin >> str_buffer >> str_pattern;
    StringPattern pattern(str_pattern);
    int location = pattern.match(str_buffer);
    if (location != -1) {
        cout << "match success, location is " << location << endl;
    } else {
        cout << "match failed!" << endl;
    }
    return 0;
}