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

难度: Easy

题目描述

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.

解题思路

代码实现

解决方案

java

func strStr(haystack string, needle string) int {

	if len(needle) > len(haystack) {
		return -1
	}
	if len(needle) == len(haystack) && needle != haystack {
		return -1
	}
 
        pi := make([]int,len(needle))

        for i,j:=1,0;i<len(needle);i++{
            for j>0 &&needle[i]!=needle[j]{
                j=pi[j-1]
            }
            if needle[i]==needle[j]{
                j++
            }
            pi[i] = j
        }

        for i,j:=0,0;i<len(haystack);i++{
            for j>0&&haystack[i]!=needle[j]{
                j=pi[j-1]
            }
            if(haystack[i]==needle[j]){
                j++
            }
            if(j==len(needle)){
                return i-len(needle)+1
            }
        }

        return -1


}