[leetcode] 28. Implement strStr() 解题报告

题目链接：https://leetcode.com/problems/implement-strstr/

Implement strStr().

Returns the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.

思路：按照简单的做可以很简单，就是找一个字符串中是否包含另一个字符串，并返回其位置。按照复杂的做那么就是KMP了, 所以 看出题人的意思了.

代码如下：

class Solution {
public:
    int strStr(string haystack, string needle) {
        int i = -1, len1 = haystack.size(), len2 = needle.size();
        while(++i <= len1-len2)
        {
            string tem = haystack.substr(i, len2);
            if(tem == needle) return i;
        }
        return -1;
    }
};

KMP

class Solution {
public:
    int strStr(string haystack, string needle) {
        int i, k = 0, len1 = haystack.size(), len2 = needle.size();
        if(len2==0 || haystack == needle) return 0;
        vector index(len2, 0);
        for(i = 1; i < len2; i++)
        {
            while(k>0 && needle[i]!=needle[k]) k = index[k-1];
            index[i] = (k+=needle[i]==needle[k]);
        }
        for(i = 0, k = 0; i < len1; i++)
        {
            while(k>0 && haystack[i]!=needle[k]) k = index[k-1];
            k += needle[k] == haystack[i];
            if(k == len2) return i-len2+1;
        }
        return -1;
    }
};