ImplementstrStr().java

/*
 Author:     King, wangjingui@outlook.com
 Date:       Dec 17, 2014
 Problem:    Implement strStr()
 Difficulty: Easy
 Source:     https://oj.leetcode.com/problems/implement-strstr/
 Notes:
 Implement strStr().
 Returns a pointer to the first occurrence of needle in haystack, or null if needle is not part of haystack.

 Solution: 1. Check in the haystack one by one. If not equal to needle, reset the pointer.(TLE)
           2. Classice KMP solution.
           3. Simplified RK Soluiton. Thanks for [wenyuanhust, wenyuanhust@gmail.com]
 */

public class Solution {
    public int strStr_1(String haystack, String needle) {
        int sLen = haystack.length(), tLen = needle.length();
        if(tLen == 0) return 0;
        if (haystack==null || needle==null || sLen==0) return -1;
        int i = 0, j = 0;
        while (i < sLen) {
            for (j = 0; j < tLen && (i+j) < sLen && haystack.charAt(i+j) == needle.charAt(j); ++j);
            if (j == tLen) return i;
            ++i;
        }
        return (int)-1;
    }
    void getNext(String T, int[] next){
        int i=0, j=-1;
        next[0]=-1;
        int n = next.length;
        while(i < n - 1){
            while(j>-1&&T.charAt(j)!=T.charAt(i)) j = next[j];
            ++i; ++j;
            if(i < n - 1 && j < n - 1 && T.charAt(j)==T.charAt(i)) next[i]=next[j];
            else next[i]=j;
        }
    }
    public int strStr_2(String haystack, String needle) {
        int sLen = haystack.length(), tLen = needle.length();
        if(tLen == 0) return 0;
        if (haystack==null || needle==null || sLen==0) return -1;
        int[] next = new int[tLen+1];
        getNext(needle, next);
        int i = 0, j = 0;
        while (i < sLen) {
            while (j > -1 && needle.charAt(j) != haystack.charAt(i)) j = next[j];
            ++i; ++j;
            if (j == tLen) return i - j;
        }
        return -1;
    }
    public int strStr_3(String haystack, String needle) {
        int sLen = haystack.length(), tLen = needle.length();
        if (tLen == 0) return 0;
        if (haystack==null || needle==null || sLen==0 || sLen < tLen) return -1;
        long fh = 0, fn = 0;
        int head = 0, tail = tLen - 1;
        for (int i = 0; i < tLen; ++i) {
            fh += haystack.charAt(i);
            fn += needle.charAt(i);
        }
        while (tail < sLen) {
            if (fn == fh) {
                int i = 0;
                while (i < tLen && needle.charAt(i) == haystack.charAt(i + head)){
                    ++i;
                }
                if (i == tLen) return head;
            }
            if (tail == sLen - 1) return -1;
            fh -= haystack.charAt(head++);
            fh += haystack.charAt(++tail);
        }
        return -1;
    }
}