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;
    }
};

代码地址

https://github.com/f304646673/leetcode/tree/main/28-Find-the-Index-of-the-First-Occurrence-in-a-String