[LintCode] strStr [KMP & brute force]

Problem

For a given source string and a target string, you should output the first index(from 0) of target string in source string.

If target does not exist in source, just return -1.

Note

我终于找到了比较好的KMP算法。
http://alice-alicesspace.blogspot.com/2015/07/strstr-kmp-solution-java.html

Solution

class Solution {
    public int strStr(String source, String target) {
        //write your code here
        if (source == null | target == null) {
            return -1;
        }
        int i, j;
        for (i = 0; i < source.length() - target.length() +1; i++) {
            for (j = 0; j < target.length(); j++) {
                if (source.charAt(i+j) != target.charAt(j)) {
                break;
                }
        }
        if (j == target.length()) {
            return i;
        }
        }
        return -1;
        }
}

KMP算法不如也贴出来。

class Solution {
    public int strStr(String source, String target) {
        //write your code here
        if (source == null || target == null) {
            return -1;
        }
        if (target.length() == 0) {
            return 0;
        }
        
        int[] prefix = new int[target.length()];
        int k = 0;
        for (int i = 1; i < target.length(); i++) {
            while (k > 1 && target.charAt(k) != target.charAt(i)) {
                k = prefix[k-1];
            }
            if (target.charAt(i) == target.charAt(k)) k++;
            prefix[i] = k;
        }
        k = 0;
        for (int i = 0; i < source.length(); i++) {
            while (k > 1 && source.charAt(i) != target.charAt(k)) {
                k = prefix[k-1];
            }
            if (target.charAt(k) == source.charAt(i)) {
                k++;
            }
            if (k == target.length()) {
                return i - k + 1;
            }
        }
        return -1;
    }
}