[LeetCode]Implement strStr()

最新推荐文章于 2019-07-15 16:09:50 发布

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最新推荐文章于 2019-07-15 16:09:50 发布

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分类专栏： LeetCode

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本文深入讲解了KMP算法的原理及实现，包括如何生成跳转表(next数组)以及如何进行字符串匹配搜索。通过两个不同的C++类实现了KMP算法，并提供了详细的代码注释帮助理解。

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class Solution {
//kmp O(n+m)
//1. search part: 
//needle[j] != neddle[i], then get j = next[j], do not let i decrease at any time
//2. generate jump table of needle: 
//get next array by matching needle string and needle+1 string
//two special state of next array, next[0] = -1 and next[1] = 0
public:
	void generate_kmp_table(char* needle, vector<int>& next)
	{//模式字符串与自身错位匹配，得到next
		int len = strlen(needle);
		if(0 == len) return;
		next.resize(len);
		int i = 0;
		int j = -1;
		next[0] = -1;
		while(i < len-1)
		{
			if(j == -1 || needle[i] == needle[j])
			{
				i++;
				j++;
				next[i] = j;
			}
			else j = next[j];
		}
	}
    char *strStr(char *haystack, char *needle) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
		
		int len1 = strlen(haystack);
		int len2 = strlen(needle);
		vector<int> next;
		generate_kmp_table(needle, next);
		//search
		int i = 0;
		int j = 0;
		while(i < len1 && j < len2)
		{
			if(j == -1 || haystack[i] == needle[j])
			{
				i++;
				j++;
			}
			else j = next[j];
		}
		if(j == len2) return &haystack[i-j];
		else return NULL;
    }
};

second time

class Solution {
public:
    void getJumpTable(char* str, vector<int>& jump)
    {
        int len = strlen(str);
        if(len == 0) return;
        jump.resize(len, 0);
        int i = 0;
        int j = -1;
        jump[0] = -1;
        while(i < len-1)
        {
             if(j == -1 || str[i] == str[j])
             {
                 i++;
                 j++;
                 jump[i] = j;
             }
             else j = jump[j];
        }
    }
    char *strStr(char *haystack, char *needle) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        int len1 = strlen(haystack);
        int len2 = strlen(needle);
        //if(len1 == 0 || len2 == 0) return NULL;
        vector<int> jump;
        getJumpTable(needle, jump);
        int i = 0;
        int j = 0;
        while(i < len1 && j < len2)
        {
             if(j == -1 || haystack[i] == needle[j])
             {
                 i++;
                 j++;
             }
             else j = jump[j];
        }
        if(j == len2) return &haystack[i-len2];
        else return NULL;
    }
};