28. Implement strStr()

Posted 2021-01-28 wentiliangkaihua

28. 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

笨方法1：

class Solution {
    public int strStr(String haystack, String needle) {
       if(needle==""||needle==null){
           return 0;
       } 
       else return haystack.indexOf(needle);
    }
}

　　直接用indexOf

方法2：

class Solution {
    public int strStr(String haystack, String needle) {
        int res = 0;
        if(needle==""||needle==null){
          return res;
       } 
        else{
            int n = needle.length();
            int i = 0;
            for(; i< haystack.length()-n; i++){
                if(haystack.substring(i,i+n).equals(needle)){
                    res = i;
                    break;
                }
            }
            if(i>=haystack.length()-n){
                res = -1;
            }
        }
        return res;
    }
}

　　奇怪的是当输入为（"","")时，leetcode会报错，但是eclipse输出还是0，不知道为什么