LeetCode Implement strStr()

Implement strStr().

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

Example 1:

Input: haystack = "hello", needle = "ll"
Output: 2

Example 2:

Input: haystack = "aaaaa", needle = "bba"
Output: -1

class Solution {
public:
    int strStr(string haystack, string needle) {
        int len1=haystack.size();
        int len2=needle.size();
        int res=-1;
        if(len2==0) return 0;
        if(len1<len2) return res;
        for(int i=0;i<len1-len2+1;i++){
            int ok=1;
            if(haystack[i]==needle[0]){
                for(int j=1;j<len2;j++){
                    if(haystack[i+j]!=needle[j]){
                        ok=0;
                        break;
                    }
                }
                if(ok==1)
                    return i;
            }
        }
        return res;
    }
};

大神利用KMP算法实现 Very awesome

class Solution {
public:
    int strStr(string haystack, string needle) {
        int m = haystack.length(), n = needle.length();
        if (!n) return 0;
        vector<int> lps = kmpProcess(needle);
        for (int i = 0, j = 0; i < m; ) {
            if (haystack[i] == needle[j]) { 
                i++;
                j++;
            }
            if (j == n) return i - j;
            if (i < m && haystack[i] != needle[j]) {
                if (j) j = lps[j - 1];
                else i++;
            }
        }
        return -1;
    }
private:
    vector<int> kmpProcess(string& needle) {
        int n = needle.length();
        vector<int> lps(n, 0);
        for (int i = 1, len = 0; i < n; ) {
            if (needle[i] == needle[len])
                lps[i++] = ++len;
            else if (len) len = lps[len - 1];
            else lps[i++] = 0;
        }
        return lps;
    }
};