28. Implement strStr()

public int strStr(String haystack, String needle) {
  for (int i = 0; ; i++) {
    for (int j = 0; ; j++) {
      if (j == needle.length()) return i;
      if (i + j == haystack.length()) return -1;
      if (needle.charAt(j) != haystack.charAt(i + j)) break;
    }
  }
}

KMP方法

public String strStr(String haystack, String needle) {
//KMP algorithms
if(needle.equals("")) return haystack;
if(haystack.equals("")) return null;
char[] arr = needle.toCharArray();
int[] next = makeNext(arr);

for(int i = 0, j = 0, end = haystack.length(); i < end;){
if(j == -1 || haystack.charAt(i) == arr[j]){
j++;
i++;
if(j == arr.length) return haystack.substring(i - arr.length);
}
if(i < end && haystack.charAt(i) != arr[j]) j = next[j];
}
    return null;
}

private int[] makeNext(char[] arr){
int len = arr.length;
int[] next = new int[len];

next[0] = -1;
for(int i = 0, j = -1; i + 1 < len;){
if(j == -1 || arr[i] == arr[j]){
next[i+1] = j+1;
if(arr[i+1] == arr[j+1]) next[i+1] = next[j+1];
i++;
j++;
}
if(arr[i] != arr[j]) j = next[j];
}

return next;
}