闯关leetcode——28. Find the Index of the First Occurrence in a String

大纲

题目
- 地址
- 内容
解题
- 代码地址

题目

地址

https://leetcode.com/problems/find-the-index-of-the-first-occurrence-in-a-string/description/

内容

Given two strings needle and haystack, return the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.

Example 1:

Input: haystack = “sadbutsad”, needle = “sad”
Output: 0
Explanation: “sad” occurs at index 0 and 6.
The first occurrence is at index 0, so we return 0.

Example 2:

Input: haystack = “leetcode”, needle = “leeto”
Output: -1
Explanation: “leeto” did not occur in “leetcode”, so we return -1.

Constraints:

1 <= haystack.length, needle.length <= 104
haystack and needle consist of only lowercase English characters.

解题

这题就是找到第一个匹配的子串。没啥特殊的技巧，逐位对比就行了。唯一需要注意的是边界问题：

如果待查找串的长度超过原串，则没有搜索的必要，因为子串不会超过原串的长度。
搜索时不用搜索到原串的最后一位。只用搜索到原串长度减去子串长度得到的下标位。

#include <string>
using namespace std;class Solution {
public:int strStr(string haystack, string needle) {auto haystackSize = haystack.size();auto needleSize = needle.size();if (haystackSize < needleSize) {return -1;}if (needleSize == 0) {return 0;}for (auto i = 0; i < haystackSize - needleSize + 1; i++) {bool found = true;for (auto j = 0; j < needleSize; j++) {if (haystack[i+j] != needle[j]) {found = false;break;}}if (found) {return i;}}return -1;}
};