28. Implement strStr()

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本文详细解析了 LeetCode 上的经典题目——实现 strStr() 函数，该函数用于查找子字符串在主字符串中首次出现的位置。通过具体示例，如在字符串 hello 中查找 ll 的位置，以及在字符串 aaaaa 中查找不存在的子串 bba，阐述了解题思路和代码实现。

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

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().



int strStr(char * haystack, char * needle){
char* ch1, *ch2;
    int i=0,j=0,len_needle;
    len_needle=strlen(needle);
    ch1=haystack;
    ch2=needle;
    while((*ch1 != '\0') && (*ch2 != '\0')){
       
        if((*ch1) == (*ch2)){
            ch2++;
            j++;
        }
        else{
            ch2=needle;
            ch1=ch1-j;
            i=i-j;
            j=0;
        }
        ch1++;    
        i++;
    }
    if(*ch2=='\0'){
        return (i-len_needle);
    }

    return -1;
}