[LeetCode28] Implement strStr()

Implement strStr().

Returns a pointer to the first occurrence of needle in haystack, or null if needle is not part of haystack.

in Java, there is an API function name indexof(), it returns index within this string of the first occurrence of the specified substring. 

java 实现

public String strStr(String haystack, String needle) {
        if(haystack.length()<=0) return null;
        int i = haystack.indexOf(needle);
        if(i==-1) return null;
        else{
            return haystack.substring(i);
        }
    }

another solution:

public String strStr(String haystack, String needle) {
        if(haystack.length()<=0 && needle.length()>0) return null;
        for(int i=0;i<=haystack.length()-needle.length();i++){
            boolean flag = true;
            for(int j=0;j<needle.length();j++){
                if(haystack.charAt(i+j)!=needle.charAt(j)){
                    flag = false;
                    break;
                }
            }
            if(flag){
                return haystack.substring(i);
            }
        }
        return null;
    }

Below two solutions is overkill for interview.

third solution: rolling hash

http://blog.csdn.net/yanghua_kobe/article/details/8914970

fourth solution: KMP

http://www.ruanyifeng.com/blog/2013/05/Knuth%E2%80%93Morris%E2%80%93Pratt_algorithm.html

For c++

strightforward use point scan from begin to end, but this complexity is O(M*N)

char *strStr(char *haystack, char *needle) {
      if(!haystack || !needle) return NULL;
    int hlength = strlen(haystack);
    int nlength = strlen(needle);
    if(hlength<nlength) return NULL;
    for(int i=0; i<hlength-nlength+1;i++){
        char *p = &haystack[i];
        int j=0;
        for(;j<nlength;j++){
            if(*p == needle[j])
                p++;
            else
                break;
        }
        if(j == nlength)
            return &haystack[i];
    }
    return NULL;
    }

the best solution is KMP, complexity is O(N)