KMP算法

 常见的字符串匹配算法中，KMP算法是比较难懂的一个，其中的思想和实现的方法也有很多种，下面介绍一种适用比较广的情况：

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

int KMP_Match(const string &target, const string &pattern) {  
    //计算匹配串的“部分匹配表”jump[]    
    const int pLength = pattern.length();
    vector<int> jump(pLength);

    jump[0] = -1;
    for (int i = 1; i < pLength; i++){
        int index = jump[i-1];
        while (index >= 0 && pattern[i] != pattern[index+1]){
            index = jump[index];//必须判断index>= 0 否则访问时会出现-1导致失败。
        }
        if(pattern[i] == pattern[index+1])
            jump[i] = index + 1;
        else
            jump[i] = -1;
    }

    for (vector<int>::iterator ibegin = jump.begin(); ibegin != jump.end(); ibegin++){
        cout << *ibegin << " " ;
    }
    cout << endl;

    //match algorithm start  
    int pattern_index = 0, target_index = 0;  
    while (pattern_index < pLength && target_index < target.length()) {  
            if (target[target_index] == pattern[pattern_index]) {  
                    ++target_index;  
                    ++pattern_index;  
            } else if (pattern_index == 0) {  //必须先判断pattern_index == 0 的情况，此时pattern已经到了第一个字符
                                                //，无法再向前退，只能将target_index+1然后匹配。
                    ++target_index;  
            } else {  
                //jump[pattern_index - 1],因为pattern_index表示当前的不匹配的字符的位置，故需要去其-1后的jump值，
                //+1 是因为patter[jump[pattern_index-1]]的值和target[target_index-1]相同，故需要将其+1后进行比较
                    pattern_index = jump[pattern_index-1] + 1;  
            }  
    } 

    if (pattern_index == pLength) {  
            return target_index - pLength;  
    } else {  
            return -1;  
    }  
}


int main(int args, char* argv[])
{
    string pattern = "abaabcac";
    string target = "acabaabaabcacaabc";
    int index = KMP_Match(target,pattern);
    cout << index << endl;
    return 1;
}