# 28. Find the Index of the First Occurrence in a String

Given two strings `needle` and `haystack`, return the index of the first occurrence of `needle` in `haystack`, or `-1` if `needle` is not part of `haystack`.

Example 1:

``````**Input:** haystack = "sadbutsad", needle = "sad"
**Output:** 0
**Explanation:** "sad" occurs at index 0 and 6.
The first occurrence is at index 0, so we return 0.
``````

Example 2:

``````**Input:** haystack = "leetcode", needle = "leeto"
**Output:** -1
**Explanation:** "leeto" did not occur in "leetcode", so we return -1.
``````

Constraints:

• `1 <= haystack.length, needle.length <= 104`
• `haystack` and `needle` consist of only lowercase English characters.

``````class Solution {
public:
int strStr(string haystack, string needle) {
if (needle.empty()) return 0;
int m = haystack.size(), n = needle.size();
if (m < n) return -1;
for (int i = 0; i <= m - n; ++i) {
int j = 0;
for (j = 0; j < n; ++j) {
if (haystack[i + j] != needle[j]) break;
}
if (j == n) return i;
}
return -1;
}
};
``````

``````class Solution {
public:
int strStr(string haystack, string needle) {
for (int i = 0; ; ++i) {
for (int j = 0; ; ++j) {
if (j == needle.size()) return i;
if (i + j == haystack.size()) return -1;
if (needle[j] != haystack[i + j]) break;
}
}
return -1;
}
};
``````

Github 同步地址：

https://github.com/grandyang/leetcode/issues/28

Shortest Palindrome

Repeated Substring Pattern

https://leetcode.com/problems/implement-strstr/

https://leetcode.com/problems/implement-strstr/discuss/12807/Elegant-Java-solution

https://leetcode.com/problems/implement-strstr/discuss/12956/C%2B%2B-Brute-Force-and-KMP

LeetCode All in One 题目讲解汇总(持续更新中…)

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