【LeetCode】28. Implement strStr() - Java实现

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1. 题目描述：

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().

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2. 思路分析：

题目的意思是给定两个字符串haystack和needle，找出needle在haystack中第一次出现的位置，如果未出现则返回-1。

思路是两个指针遍历这两个字符串，i遍历haystack的每个字符时用j依次遍历needle的字符来进行比较从i开始的字符，如果j遍历完后完全匹配则返回i，否则继续移动i重复上述操作。

3. Java代码：

源代码：见我GiHub主页

代码：

public static int strStr(String haystack, String needle) {
    if (needle.isEmpty()) {
        return 0;
    }

    for (int i = 0; i < haystack.length(); i++) {
        int cur = i;
        for (int j = 0; j < needle.length(); j++) {

            // haystack 中剩余字符已不够，无需再比较
            if (i + needle.length() > haystack.length()) {
                return -1;
            }

            if (haystack.charAt(cur) != needle.charAt(j)) {
                break;
            }

            // 完全匹配，返回i
            if (j == needle.length() - 1) {
                return i;
            }

            cur++;
        }
    }
    return -1;
}