Implement .

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:

1 /* 2 Time: O(n^2).  3 Space: O(1). 4 */ 5  6 class Solution { 7     public int strStr(String s1, String s2) { 8         //题意确认 return 0 when needle is an empty string 9         if(s2.length() == 0) return 0;10         11         //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() + 112         for(int i = 0; i < s1.length() - s2.length() + 1; i++){13             int j = i; 14             int k = 0; 15             while( j < s1.length() && k < s2.length() && s1.charAt(j) == s2.charAt(k)){16                 j++;17                 k++;18             }19             if( k == s2.length()){20                 return i;21             } 22         }   23         return -1;24     }    25 }