实现strStr()

class Solution:
    def strStr(self, haystack: str, needle: str) -> int:
    
        # Func: 计算偏移表
        def calShiftMat(st):
            dic = {}
            for i in range(len(st)-1,-1,-1):
                if not dic.get(st[i]):
                    dic[st[i]] = len(st)-i
            dic["ot"] = len(st)+1
            return dic
        
        # 其他情况判断
        if len(needle) > len(haystack):return -1
        if needle=="": return 0
       
        # 偏移表预处理    
        dic = calShiftMat(needle)
        idx = 0
    
        while idx+len(needle) <= len(haystack):
            
            # 待匹配字符串
            str_cut = haystack[idx:idx+len(needle)]
            
            # 判断是否匹配
            if str_cut==needle:
                return idx
            else:
                # 边界处理
                if idx+len(needle) >= len(haystack):
                    return -1
                # 不匹配情况下，根据下一个字符的偏移，移动idx
                cur_c = haystack[idx+len(needle)]
                if dic.get(cur_c):
                    idx += dic[cur_c]
                else:
                    idx += dic["ot"]
            
        
        return -1 if idx+len(needle) >= len(haystack) else idx

class Solution:
    def strStr(self,haystack:str,needle:str) ->
        #计算偏移表
        def calShiftMat(st):
            dic = {}
            for i in range(len(st) -1,-1,-1):
                if not dic.get(st[i]):
                    dic[st[i]] = len(st) -i
            dic["ot"] = len(st) +1
            return dic
        #其他情况判断
        if len(needle) > len(haystack) : return -1
        if needle == "": return 0
        # 偏移表预处理
        dic = calShiftMat(needle)
        idx = 0
        
        while idx +len(needle) <= len(haystack):
            #带匹配字符串
            str_cut = haystack[idx:idx+len(needle)]
            #判断是否匹配
            

class Solution {
public:
    int strStr(string haystack, string needle) {
        int n = haystack.size(), m = needle.size();
        for (int i = 0; i + m <= n; i++) {
            bool flag = true;
            for (int j = 0; j < m; j++) {
                if (haystack[i + j] != needle[j]) {
                    flag = false;
                    break;
                }
            }
            if (flag) {
                return i;
            }
        }
        return -1;
    }
};

class Solution{

class Solution:
    def strStr(self, haystack: str, needle: str) -> int:
        a=len(needle)
        b=len(haystack)
        if a==0:
            return 0
        next=self.getnext(a,needle)
        j=-1
        for i in range(b):
            while j>=0 and needle[j+1]!=haystack[i]:
                j=next[j]
            if needle[j+1]==haystack[i]:
                j+=1
            if j==a-1:
                return i-a+1
        return -1

    def getnext(self,a,needle):
        next=['' for i in range(a)]
        j=-1
        next[0]=j
        for i in range(1,len(needle)):
            while (j>-1 and needle[j+1]!=needle[i]):
                j=next[j]
            if needle[j+1]==needle[i]:
                j+=1
            next[i]=j
        return next