LeetCode:28.Strstr

解题思路：

KMP模式匹配算法，先生成next数组，再对字符串进行匹配.

class Solution {
public:
    int strStr(string haystack, string needle) {
        int n = haystack.size(), m = needle.size();
        if(m == 0) return 0;
        haystack.insert(haystack.begin(),' ');
        needle.insert(needle.begin(),' ');
        vector<int> next(m + 1);
        //预处理next数组
        for(int i = 2, j = 0; i <= m; i++){
            while(j and needle[i] != needle[j + 1]) j = next[j];
            if(needle[i] == needle[j + 1]) j++;
            next[i] = j;
        }
        //匹配过程
        for(int i = 1, j = 0; i <= n; i++){
            while(j and haystack[i] != needle[j + 1]) j = next[j];
            if(haystack[i] == needle[j + 1]) j++;
            if(j == m) return i - m;
        }
        return -1;
    }
};

运行结果：