Implement strStr().
Returns the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.
Tags: Two Pointers, String
题意是从主串中找到子串的索引，如果找不到则返回-1，当字串长度大于主串，直接返回-1，然后我们只需要遍历比较即可。
class Solution {
public int strStr(String haystack, String needle) {
int l1 = haystack.length(), l2 = needle.length();
if (l1 < l2) return -1;
for (int i = 0; ; i++) {
for (int j = 0; ; j++) {
if (j == l2) return i;
if (i + j == l1) return -1;
if (haystack.charAt(i + j) != needle.charAt(j)) break;
}
}
}
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