028ImplementStrStr.md

Implement strStr()

Description

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().

Tags:** Two Pointers, String

思路

题意是从主串中找到子串的索引，如果找不到则返回-1，当子串长度大于主串，直接返回-1，然后我们只需要遍历比较即可。

func strStr(_ haystack: String, _ needle: String) -> Int {
    let haystackLen = haystack.count
    let needleLen = needle.count
    if haystackLen < needleLen {
        return -1
    }
    for i in 0...Int.max {
        if (i + needleLen) > haystackLen {
            return -1
        }
        for j in 0...Int.max {
            if j == needleLen {
                return i
            }
            
            let hayChar = haystack[haystack.index(haystack.startIndex, offsetBy: (i+j))]
            let needleChar = needle[needle.index(needle.startIndex, offsetBy: j)]
            if hayChar != needleChar{
                break;
            }
        }
    }
    
    return -1
}

let haystack1 = "hello", needle1 = "ll"
print("haystack: \(haystack1); needle: \(needle1); result: \(strStr(haystack1, needle1))")
let haystack2 = "aaaaa", needle2 = "bba"
print("haystack: \(haystack2); needle: \(needle2); result: \(strStr(haystack2, needle2))")

结语

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