模式匹配 KMP算法详解

/* *KMP 模式匹配算法 */#include <iostream>#include <cstring>using namespace std;/* *计算模式串的next数组 *模式串既做主串，又做模式串，进行匹配 *时间复杂度为O(m)，m为模式串的长度 */void countNext(char* strPattern, int len, int* next){int i = 0, j = -1;next[i] = j;while (i < len-1){if (j == -1){i++;j++;next[i] = j;} else if (strPattern[i] == strPattern[j]) {i++;j++;if (strPattern[i] != strPattern[j]){next[i] = j;} else {next[i] = next[j];}} else {j = next[j];}}}/* *利用KMP算法进行模式匹配 *KMP算法的时间复杂度为O(n)+O(m) = O(n+m) *其中n为主串的长度，m为模式串的长度，需要先算出next数组 *KMP算法的核心就是在搜索模式串的过程中，主串的指针不会回朔 *只会尽可能的不断向后移，避免无效的回朔，节约了时间，适合于主串 *中有较多部分匹配模式串的情况，否则如果没有什么回朔，也就不需要用KMP算法了 */int indexKMP(char* strMain, int lenMain, char* strPattern, int lenPattern, int pos, int* next){int i = pos, j = 0;while (i < lenMain && j < lenPattern){if (j == -1 || strMain[i] == strPattern[j]){i++;j++;} else {j = next[j];}}if (j >= lenPattern){return i - lenPattern;}return -1;}int main(){char* strPattern = "abce";char* strMain = "ksekabcedwfabcekf";int lenMain = strlen(strMain);int lenPattern = strlen(strPattern);int* next = new int[lenPattern];countNext(strPattern, lenPattern, next);int startPos = 5;int pos = indexKMP(strMain, lenMain, strPattern, lenPattern, startPos, next);if (pos < 0){cout<<"从字符串 "<<strMain<<" 的第 "<<startPos+1<<" 位置开始没有找到字符串 "<<strPattern<<endl;} else {cout<<"从字符串 "<<strMain<<" 的第 "<<startPos+1<<" 位置开始找到字符串 "<<strPattern<<" 在第 "<<pos+1<<" 个位置"<<endl;}delete[] next;return 0;}