记录LeetCode第七天（28. Implement strStr()和s.find和KMP）

记录LeetCode第七天（28. Implement strStr()和s.find和KMP）

1. 题目

Implement strStr().
Return the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.

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().

2. 例子

Example 1:

Input: haystack = “hello”, needle = “ll”
Output: 2

Example 2:

Input: haystack = “aaaaa”, needle = “bba”
Output: -1

3. 代码

class Solution {
public:
    int strStr(string haystack, string needle) {
        int m = haystack.size(),n = needle.size();
        for(int i=0 ;i<=m-n;i++)//i是haystack的索引，j是needle的索引，
        {
            int j=0;
            while(haystack[i+j]==needle[j] && j<n)//如果匹配，判断下一对直到有不匹配或needle全部判断完。
            {
                j++;
            }
            if(j==n) return i;//如果needle全部匹配完，返回第一个匹配的haystack索引。
        }
        return -1；//如果到最后也没匹配完成，就返回-1；
    }
};

其实就是find() 的复现

class Solution {
public:
    int strStr(string haystack, string needle) {
        if(needle.empty()) return 0;
        int n = haystack.find(needle);
        return n;
    }
};

4. find() 和KMP

str1.find(str2)在字符串str1中找字符串str2 ，如果找到str2返回str1第一个匹配字符的索引，如果不匹配返回-1。
KMP 算法：
https://blog.youkuaiyun.com/v_july_v/article/details/7041827
优化后还没有看懂。。。就以后补。。。