[刷题防痴呆] 0028 - 实现 strStr() (Impl

题目地址

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

题目描述

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
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().

思路

• for循环即可. i在source中移动, j在target中移动. 如果source中在i+j位置的char不等于target中在j位置的char. i++, 开始下一轮计算.
• 如果j已经等于target的length, 说明已经找到了. 返回i即可.
• 注意边界条件, 如果一直没找到, return -1.

关键点

• for循环从0到haystack.length() - needle.length() + 1.

代码

• 语言支持：Java

class Solution {
    public int strStr(String haystack, String needle) {
        if (haystack == null || needle == null) {
            return -1;
        }
        for (int i = 0; i < haystack.length() - needle.length() + 1; i++) {
            int j = 0;
            for (; j < needle.length(); j++) {
                if (haystack.charAt(i + j) != needle.charAt(j)) {
                    break;
                }
            }
            if (j == needle.length()) {
                return i;
            }
        }
        
        return -1;
    }
}