LeetCode-28. 实现 strStr()（Implement strStr()）——KMP算法

KMP算法

class Solution {
private:
	vector<vector<int>> dp;
public:
    int strStr(string haystack, string needle) {
    	if(needle.size() == 0) {
    		return 0;
    	}
    	int j = 0;
    	KMP(needle);
    	for(int i = 0; i < haystack.size(); ++i) {
    		//根据状态机更改状态
    		j = dp[j][haystack[i]];
    		if(j == needle.size()) {
    			return i - needle.size() + 1;
    		}
    	}    
    	return -1;
    }
    /**
    构建状态机
    **/
    void KMP(string needle) {
    	dp.resize(needle.size(), vector<int>(256));
    	dp[0][needle[0]] = 1;
    	int x = 0;
    	for (int i = 1; i < needle.size(); ++i)
    	{
    		for(int c = 0; c < 256; ++c) {
    			if(needle[i] == c){//推进
    				dp[i][c] = i + 1; 
    			}else{//重启
    				dp[i][c] = dp[x][c];
    			}
    		}
    		//更新影子
    		x = dp[x][needle[i]];
    	}
    }
};

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