28. Implement strStr()

题意

求p串在s串中的出现位置

思路

扩展KMP算法，时间复杂度：O(n)

代码

class Solution {
public:
    int strStr(string haystack, string needle) {
        if (needle.size() == 0) return 0;
        int next[100005];
        next[0] = -1;
        int i = 0, j = -1;
        while (i < needle.size())
        {
            if (j == -1 || needle[i] == needle[j]) 
            {
                next[++i] = ++j;
                if (needle[i] == needle[next[i]]) next[i] = next[next[i]];
            }
            else
                j = next[j];
        }
        
        i = 0, j = 0;
        while (i < haystack.size())
        {
            if (j == -1 || haystack[i] == needle[j])
            {
                ++i, ++j;
                if (j == needle.size())
                    return i - needle.size();
            }
            else
                j = next[j];
            
        }
        return -1;
    }
};