题目

LeetCode
LeetCode-cn

Implement strStr().

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

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().

Example 1:
Input: haystack = "hello", needle = "ll"
Output: 2

Example 2:
Input: haystack = "aaaaa", needle = "bba"
Output: -1

Example 3:
Input: haystack = "", needle = ""
Output: 0
 
Constraints:
0 <= haystack.length, needle.length <= 5 * 104
haystack and needle consist of only lower-case English characters.

题解

难度简单。
这道题就是说要找到needle在haystack第一个出现的位置，如果没有出现就返回-1。

解法一：暴力法

//Go
func strStr(haystack string, needle string) int {
    //考虑特殊情况
    if len(haystack) == 0 && len(needle) == 0 {
        return 0
    }
    if len(haystack) == 0 {
        return -1
    }
    if len(needle) == 0 {
        return 0
    }
    if len(haystack) < len(needle) {
        return -1
    }
    len_h := len(haystack)  //获取haystack字符串的长度
    len_n := len(needle)  //获取needle字符串的长度
    for i:=0;i<len_h-len_n+1;i++ {
        j := 0;  //子串每次都要重头开始遍历
        for ;j<len_n;j++ {
            if (haystack[i+j] != needle[j]) {
                break;
            }
        }
        if (j == len_n) {
            return i;
        }
            
    }

    return -1; 
}

执行结果：

leetcode-cn：
执行用时：0 ms, 在所有 Go 提交中击败了100.00%的用户
内存消耗：2.2 MB, 在所有 Go 提交中击败了64.54%的用户

leetcode：
Runtime: 0 ms, faster than 100.00% of Go online submissions for Implement strStr().
Memory Usage: 2.3 MB, less than 100.00% of Go online submissions for Implement strStr().

参考资料

Golang中的内置函数strings.Index也可以实现，可以参考它的源码实现。

//Go
import "strings"
func strStr(haystack string, needle string) int {
    return strings.Index(haystack,needle)
}