README.md

Implement strStr()

Description

Implement strStr().

Return the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.

Example 1:

Input: haystack = "hello", needle = "ll"
Output: 2

Example 2:

Input: haystack = "aaaaa", needle = "bba"
Output: -1

Clarification:

What should we return when needle is an empty string? This is a great question to ask during an interview.

For the purpose of this problem, we will return 0 when needle is an empty string. This is consistent to C's strstr() and Java's indexOf().

Tags:** Two Pointers, String

思路

题意是从主串中找到子串的索引，如果找不到则返回-1，当子串长度大于主串，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++) {
            if (i + l2 > l1) return -1;
            for (int j = 0; ; j++) {
                if (j == l2) return i;
                if (haystack.charAt(i + j) != needle.charAt(j)) break;
            }
        }
    }
}

结语

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