[leetcode]28. Implement strStr()实现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.

题意：

实现strStr() ： Returns a pointer to the first occurrence of str2 in str1, or a null pointer if str2 is not part of str1.

Solution1：Two Pointers, finding substring[i...j] in str1，such that it equals str2

code:

/*
Time: O(n^2). 
Space: O(1).
*/

class Solution {
    public int strStr(String s1, String s2) {
        //题意确认 return 0 when needle is an empty string
        if(s2.length() == 0) return 0;
        
        //for(int i = 0; i < s1.length(); i++){ 确保s1中含有s2，则扫s1的指针的范围可以缩小到s1.length() - s2.length() + 1
        for(int i = 0; i < s1.length() - s2.length() + 1; i++){
            int j = i; 
            int k = 0; 
            while( j < s1.length() && k < s2.length() && s1.charAt(j) == s2.charAt(k)){
                j++;
                k++;
            }
            if( k == s2.length()){
                return i;
            } 
        }   
        return -1;
    }    
}